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8061  'II 

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A  GUIDE  FOR  THE  DISSECTION 

OF  THE  DOGFISH 
(SQUALUS  ACANTHIAS) 


By 

LAWRENCE  E.  GRIFFIN 
Professor  of  Biology  in  Reed  College 

THIRD  EDITION 

Portland,   Oregon 
1922 


Copyright,  1922 

by 
LAWRENCE    E.  GRIFFIN 


BiOi 
LIB   - 
Q 

A  Guide  for  the  Dissection 
of  the  Dogfish 

The  small  sharks  which  abound  along  the  coasts  of  the  United 
States  are  commonly  called  "dogfish"  by  fishermen  and  others.  The 
"dogfish"  of  inland  waters  belongs  to  an  entirely  different  group. 
Two  species  of  sharks  are  caught  in  numbers  and  used  in  laboratory 
work,  the  "spiny  dogfish"  (Squalus  acanthias)  and  the  "smooth 
dogfish"  (Eugaleus  galeus).  The  first'  is  easily  distinguished  by 
the  sharp  spine  in  front  of  each  dorsal  fin.  Squalus  acanthias  is 
often  referred  to  under  the  synonym  Acanthias  vulgaris,  while  Eu- 
galeus galeus  is  more  frequently  named  either  Mustelus  canis  or 
Galeus  canis.  The  histories  of  these  names  and  systematic  de- 
scriptions of  the  species  are  contained  in  Samuel  Carman's  Mono- 
graph on  the  Elasmobranchs. 

Several  sizes  of  dogfish  are  furnished  by  dealers.  We  consider 
it  best  to  purchase  large,  fully  developed  specimens.  The  small 
specimens  may  be  a  little  more  convenient  to  handle,  but  the  large 
ones  have  the  important  advantage  of  being  sexually  mature,  while 
blood  vessels  and  nerves  are  dissected  better  in  large  than  in  small 
specimens.  Also,  a  number  of  structures  are  very  different  in  ma- 
ture animals  from  their  condition  in  young  ones.  Dealers  should 
be  requested  to  furnish  fish  with  fins  and  tails  complete  instead  of 
trimmed.  It  is  an  advantage  to  issue  to  the  class  equal  numbers  of 
both  sexes. 

An  entire  specimen  and  an  extra  head  may  be  required  by  each 
student  for  a  thorough  dissection.  It  would  be  better  if  the  head 
were  cut  off  just  behind  the  pectoral  fins,  instead  of  in  front  of  them 
as  is  usually  done,  so  that  the  vagus  and  hypobranchial  nerves  may 
be  dissected  more  completely. 

The  spiny  dogfish,  which  is  the  particular  subject  of  this  guide, 
is  the  species  most  frequently  supplied  to  laboratories.  However, 
the  spiny  and  smooth  dogfishes  are  so  much  alike  that  the  latter 
may  be  easily  dissected  with  these  directions.  Where  marked  differ- 
ences between  the  forms  exist  the  structure  of  Eugaleus  is  described 
separately. 

The  student  of  anatomy  should  realize  that  dissection  is  for  the 
purpose  of  enabling  him  to  see  for  himself  the  structures  which 
exist,  and  that  no  dissection  is  satisfactory  until  the  anatomical 
arrangements  mentioned  in  the  text  can  be  completely  demonstrated 
in  his  specimen. 

The  importance  of  knowing  the  structure  of  the  elasmobranch 
is  so  great  in  comparative  anatomy  and  embryology  that  it  is  worth 
while  to  make  a  thorough  dissection  of  one  of  this  class.  As  the 
dogfish  is  frequently  the  first  major  vertebrate  form  to  be  studied 
in  detail,  these  directions  have  been  written  to  conform  to  the  needs 
of  the  student  who  is  beginning  comparative  anatomy.  The  arrange- 
ment of  sections  in  this  guide  is  intended  to  permit  the  omission  of 
some  which  it  may  not  be  considered  desirable  to  include  in  the 
work  of  a  class. 


48347 


EXTERNAL  CHARACTERS 

The  spindle-shaped  body  tapers  from  near  the  middle  toward 
both  head  and  tail;  the  head  is  flattened  on  both  the  dorsal  and 
ventral  sides,  while  the  remainder  of  the  body  is  nearly  round,  with 
a  lateral  compression  which  is  not  pronounced  except  in  the  caudal 
portion. 

The  general  color  of  the  back  and  sides  is  gray;  darkest  above, 
where  the  skin  is  spotted  with  scattered,  small,  round,  light  spots. 
The  color  of  the  upper  parts  shades  into  the  yellow  white  of  the 
ventral  surface. 

Can  head,  trunk,  and  tail  regions  be  distinguished?  If  so,  what 
characters  determine  the  extent  of  each? 

A  little  above  the  middle  of  the  side  of  the  body  is  the  lateral 
line,  (distinguished  partly  by  color,  partly  by  being  slightly  ele- 
vated), which  extends  from  the  back  of  the  head  to  the  tail.  Cut 
through  the  skin  across  the  lateral  line  at  several  points  along  the 
body  and  notice  the  canal  which  lies  in  the  dermis  under  the  lateral 
line.  This  is  the  lateral  line  canal,  which  opens  to  the  surface  by 
numerous  pores  (too  minute  to  be  seen),  and  contains  a  series  of 
special  sense  organs  along  its  dorsal  and  inner  surfaces.  Near  the 
base  of  the  caudal  fin  the  lateral  line  canal  passes  into  a  groove 
which  continues  the  lateral  line  to  within  a  short  distance  of  the 
edge  of  the  fin.  The  lateral  line  canal  in  its  development  begins 
as  a  groove  along  the  side  of  the  body  which  becomes  closed  by  the 
fusion  of  its  edge  except  in  this  terminal  portion. 

The  open  groove  does  not  appear  in  Eugaleus. 

In  the  midline  of  the  body  are  two  triangular  dorsal  fins,  each 
attached  to  the  body  for  about  half  its  length.  The  basal  portion 
of  each  is  thick  and  muscular,  and  contains  supporting  cartilages 
embedded  in  the  muscles.  The  remainder  of  the  fin  is  flexible  and 
semi-transparent,  horny  finrays  being  faintly  visible  between  the 
layers  of  skin.  In  front  of  each  fin  is  a  strong  spine  which  seems 
to  serve  both  as  a  cutwater  and  a  weapon  of  defense.  (Eugaleus 
has  no  spines.) 

The  broad,  paired,  pectoral  fins,  having  the  general  characteris- 
tics of  the  dorsals,  spring  from  the  ventral  edges  of  the  body  just 
back  of  the  head.  A  hard  bar  of  cartilage  connecting  the  bases  of 
the  pectoral  fins  can  be  felt  through  the  skin  of  the  ventral  surface 
of  the  body.  This  is  the  ventral  part  of  the  pectoral  girdle. 

Farther  back,  and  also  on  the  ventral  surface,  are  the  paired 
pelvic  fins.  The  pelvic  girdle  can  be  felt  through  the  skin  between 
the  bases  of  these  fins.  If  the  specimen  is  a  male,  it  will  have  a 
fingerlike  process  projecting  backward  from  the  base  and  along  the 
inner  side  of  each  pelvic  fin.  These  organs,  which  attain  a  consid- 
erable size  in  adults,  are  modified  portions  of  the  pelvic  fin  used  as 
copulatory  organs.  They  are  named  variously  claspers,  myxop- 
terygia,  or  pterygopodia.  A  groove  runs  along  the  dorsal  side  of 
the  clasper  from  the  tip  to  near  the  base,  where  it  opens  into  a  long 
sac  (glandula  pterygopodia)  extending  some  distance  in  front  of 
the  pelvic  fins  just  within  the  skin  of  the  ventral  surface.  By  feeling 
a  cartilaginous  axis  of  the  clasper  can  be  discovered,  which  extends 
to  the  tip  of  the  organ.  On  the  dorsal  surface  and  near  the  end  of 
the  clasper  is  a  sharp  grooved  spine  on  the  outer  side,  and  a  strongly 
recurved  hook  on  the  inner  side,  both  almost  hidden  by  a  flap  pro- 
jecting from  the  inner  edge  of  the  groove.  This  flap  is  stiffened  by 
a  series  of  small  cartilages  similar  to  the  radial  cartilages  in  the 
bases  of  the  fins. 


The  caudal  fin  is  asymmetrical,  extending  along  the  dorsal  and 
ventral  edges  of  the  posterior  end  of  the  body.  Observe  the  upward 
bend  of  the  vertebral  column  which  occurs  in  the  tail;  it  is  this 
character  which  marks  the  heterocercal  type  of  tail. 

Eugaleus  has  a  median  ventral,  or  anal,  fin  a  short  distance  anterior  to  the 
caudal  fin. 

The  mouth  is  a  broad  transverse  slit  upon  the  ventral  surface 
of  the  head.  The  cartilaginous  jaws  can  be  seen  and  felt  just  within 
the  mouth.  Both  upper  and  lower  jaws  are  armed  with  rows  of  flat, 
sharply  pointed  teeth.  Study  their  arrangement  and  approximate 
number.  Jaws  of  previously  dissected  specimens  should  be  exam- 
ined under  a  dissection  miscroscope.  The  exact  form  and  arrange- 
ment of  the  functional  teeth  can  then  be  ascertained  easily,  and  an 
examination  of  the  inner  surface  of  either  jaw  will  disclose  several 
rows  of  developing  teeth.  As  the  young  teeth  develop  they  move,  a 
row  at  a  time,  into  position  on  the  edge  of  the  jaw;  the  oldest  teeth, 
occupying  the  outer  row,  are  shed  at  about  the  same  time. 

The  upper  jaw  is  partly  overhung  by  a  lip-like  fold  of  skin.  At 
each  side  of  the  mouth  is  a  pocket,  directed  obliquely,  having  no 
communication  with  the  mouth.  These  labial  pockets  provide  places 
for  the  labial  cartilages  (which  can  be  felt  along  the  medial  edges) 
when  the  mouth  is  closed,  and  also  afford  freedom  of  motion  to  the 
mandible.  Cut  along  the  inner  edge  of  the  labial  pocket  and  ex- 
pose the  cartilages  for  examination.  (The  labial  pocket  of  Euga- 
leus is  much  smaller,  and  in  front  of  the  corner  of  die  mouth  rather 
than  lateral  to  it.  The  two  cartilages  are  completely  separated  from 
each  other,  the  posterior  one  scarcely  reaching  the  pocket.) 

In  front  of  the  mouth  are  the  nostrils,  their  apertures  apparently 
divided  by  projecting  flaps  of  the  anterior  margin.  Explore  the 
cavity  of  the  nostril  with  a  probe  to  get  a  good  idea  of  its  size  and 
form. 

Between  the  pelvic  fins  is  the  cloaca,  a  large  depression  into 
which  open  the  alimentary  canal,  the  excretory  and  genital  ducts, 
and  the  abdominal  pores.  The  opening  of  the  alimentary  canal,  the 
anus,  is  at  the  anterior  end  of  the  cloaca.  In  preserved  specimens 
part  of  the  intestine  is  frequently  everted  through  the  anus.  A  large 
fleshy  process,  bearing  a  pore  at  its  tip,  projects  from  the  dorsal  wall 
of  the  cloaca.  In  the  male  this  is  the  uro genital  papilla;  in  the 
female  the  urinary  papilla.  In  the  female  a  genital  pore,  the  open- 
ing of  the  oviduct,  is  found  on  either  side  of  the  papilla.  An 
abdominal  pore,  leading  into  the  abdominal  cavity,  is  found  on 
each  side  of  the  cloaca  at  the  posterior  margin.  These  are  frequently 
closed  in  young  specimens. 

The  cloaca  of  Eugaleus  has  a  comparatively  small  opening  upon  the  ventral 
surface,  which  must  be  enlarged  before  the  parts  described  can  be  seen  well. 

The  laterally  placed  eyes  are  without  lids;  observe  the  consider- 
able difference  in  the  amount  of  curvature  of  the  dorsal  and  ventral 
margins  of  the  eye. 

In  Eugaleus  there  is  a  fold  of  skin  stretched  across  the  lower  part  of  the 
eye  which  serves  as  an  eye-lid,  and  corresponds  to  the  so-called  "third  eye-lid" 
or  nictitating  membrane  of  other  vertebrates. 

On  each  side  of  the  neck  are  five  vertical  gill-clefts,  each  leading 
into  a  large  gill-pouch  which  communicates  with  the  pharynx  by  an 
internal  opening.  Pass  a  probe  through  a  gill-cleft  into  the  mouth. 

Back  of  each  eye  is  a  small  aperture,  the  spiracle;  explore  this 
cavity  with  a  probe.  The  spiracle  is  to  be  considered  a  gill-cleft 
moved  forward  upon  the  head  and  largely,  though  not  entirely,  de- 
prived of  its  respiratory  functions. 

In  the  center  of  the  dorsal  surface  of  the  head,  between  the 


4 

spiracles,  are  two  pores,  the  external  openings  of  the  endolymphatic 
ducts  which  communicate  with  the  internal  ear.  Large  numbers  of 
smaller  pores  can  be  found  on  all  surfaces  of  the  head,  some  in 
groups,  some  arranged  linearly,  many  scattered.  Most  of  those 
arranged  in  lines  lead  into  the  sensory  canal  system  which  continues 
from  the  lateral  line  canal  upon  the  head,  while  the  majority  of 
the  others  belong  to  a  separate  type  of  sense  organs,  the  ampullae 
of  Lorenzini. 

Make  a  cut  encircling  the  pores  of  the  endolymphatic  ducts  and 
close  to  them.  Do  not  remove  this  piece  of  skin.  From  it  make  a 
median  incision  forward  to  the  tip  of  the  snout  and  back  as  far  as 
the  level  of  the  first  gill  slit.  Starting  at  this  incision  work  the 
skin  off  from  the  tissues  beneath  it.  This  must  be  a  careful,  close 
dissection.  When  the  lateral  line  is  reached  it  will  be  seen  that  the 
lateral  line  canal  is  continued  upon  the  head  and  is  joined  by  sev- 
eral others.  By  looking  through  the  loosened  skin  toward  the  light 
the  pores  can  be  seen  which  lead  from  the  canals  to  the  surface.  In 
the  hollow  of  the  skull  in  front  of  and  above  the  eye  is  a  large 
group  of  tubules  which  open  through  the  pores  so  conspicuous  at 
this  point.  At  the  internal  end  of  each  tubule  is  a  slight  enlarge- 
ment, of  denser  tissue,  with  which  a  delicate  nerve  strand  can  often 
be  seen  connected.  The  nerve  strands  can  be  traced  to  a  large 
nerve  passing  above  the  eye  and  distributed  to  the  snout.  These 
tubules  are  the  ampullae  of  Lorenzini.  This  group  of  ampullae, 
which  may  be  called  the  dorsal  group,  is  quite  definitely  demarked. 
It  will  be  noted  that  the  inner  ends  of  the  ampullae  are  grouped  in 
a  much  smaller  area  than  their  pores. 

Between  the  spiracle  and  the  first  gill  slit  will  be  found  a  lateral 
group  of  similar  organs.  Notice  the  arrangement  of  their  tubules 
and  pores.  Under  the  snout  are  two  groups  of  ampullae  on  each 
side  of  the  midline.  The  inner  ventral  group  is  separated  from  the 
outer  ventral  group  by  the  lateral  bar  of  the  rostral  cartilage.  Some 
of  the  tubules  of  the  outer  ventral  group  will  be  found  to  extend  to 
pores  situated  at  the  sides  of  and  behind  the  mouth. 

An  adult  fish  possesses  from  1200  to  1900  ampullae  of  Loren- 
zini. Their  function  is  not  well  understood,  but  it  has  been  sug- 
gested that  they  are  organs  responsive  to  stimuli  of  pressure,  either 
of  currents  or  water,  or  resulting  from  depth,  or  even  of  deep 
tones. 

The  system  of  sensory  canals  consists  of  the  following  members 
on  each  side  of  the  head: — 

A  supraorbitaly  passing  above  the  eye  to  the  end  of  the  snout 
and  bending  back  on  the  ventral  surface  to  join  the  infra-orbital. 

An  infra-orbital.,  which  branches  off  from  the  supra-orbital  and 
passes  ventrad  between  the  eye  and  the  spiracle,  then  turns  forward 
along  the  ventral  margin  of  the  orbit,  and  finally  bends  toward  the 
mid-line  and  extends  to  the  tip  of  the  snout. 

A  hyomandibular,  which  leaves  the  infra-orbital  below  the  eye 
and  runs  back  beyond  the  angle  of  the  mouth. 

A  short  mandibular,  on  the  mandible  close  to  the  angle  of  the 
mouth,  which  is  not  connected  with  the  other  canals. 

The  canal  systems  of  the  right  and  left  sides  are  connected  by  a 
supra-temporal  canal  just  behind  the  endolymphatic  pores,  and 
often  by  an  anastomosis  of  the  infra-orbital  canals  in  front  of  the 
mouth. 

On  some  specimens  two  crescentic  rows  of  pores  can  be  found 
between  the  bases  of  the  pectoral  fins,  which  represent  a  third  type 


of  sensory  organ,  the  pit  organs,  closely  related  genetically  to  the 
sensory  canals.  Two  longer  lines  of  pit  organs,  (the  mandibular 
pit  organs),  will  be  found  a  short  distance  behind  the  mouth.  Sim- 
ilar pit  organs  are  found  in  front  of  the  endolymphatic  pores,  and 
above  the  anterior  part  of  the  lateral  line. 

Except  for  a  few  small  areas  the  entire  surface  of  the  body  is 
covered  with  small,  sharp-pointed  denticles  (placoid  scales) .  Each 
consists  of  a  diamond-shaped  basal  plate  embedded  in  the  dermis, 
from  which  projects  a  leaf -like,  backward  directed  spine.  A  piece 
of  skin  should  be  removed  and  examined  under  a  low  magnifica- 
tion. The  dermis  is  so  dense  and  pigmented  that  the  basal  plate  is 
not  easily  studied  without  further  manipulation.  For  this  purpose 
boil  a  piece  of  skin  in  5%  caustic  potash  solution  until  it  is  softened, 
but  not  till  it  disintegrates.  Then  clear  it  in  glycerine.  Examine 
the  individual  denticles  under  a  higher  magnification.  The  den- 
ticle consists  of  dentine,  the  spine  being  of  a  much  denser  structure 
than  the  base.  The  teeth  and  the  large  spines  of  the  fins  and  claspers 
are  also  composed  of  dentine  and  may  be  considered  as  modified 
placoid  scales.  Denticles,  teeth,  and  spines  are  covered  with  a 
shiny,  enamel-like  layer  which,  however,  does  not  appear  to  be  true 
enamel  such  as  covers  the  teeth  of  higher  vertebrates.  The  shape 
of  the  scales  and  their  closeness  vary  on  different  regions  of  the 
body,  and  there  are  certain  regions  entirely  free  from  them,  namely, 
back  of  the  dorsal,  pectoral  and  pelvic  fins,  the  medio-dorsal  sur- 
faces of  the  claspers,  inside  the  upper  lip  and  the  labial  pockets. 

DISSECTION  OF  THE  ABDOMINAL  VISCERA 

Place  the  dogfish  on  its  back  and,  commencing  at  the  middle  of 
the  abdomen,  make  an  incision  through  the  body  wall  a  quarter  of 
an  inch  to  one  side  of  the  midline.  Carry  this  forward  to  the  pec- 
toral girdle  and  backward  through  the  pelvic  girdle  to  the  cloaca; 
not,  however,  cutting  the  wall  of  the  cloaca. 

A  large  vein,  the  lateral  vein,  runs  along  the  inner  surface  of 
each  lateral  wall  of  the  abdomen.  After  identifying  these,  cut 
through  the  body  wall  transversely  on  both  sides  of  the  abdomen  at 
the  level  of  the  posterior  attachment  of  the  pectoral  fin  as  far  as 
the  lateral  vein.  Turn  the  flaps  outward  and  fasten. 

The  coelom  or  body-cavity  consists  of  two  portions,  the  abdom- 
inal and  pericardial  cavities.  The  abdominal  cavity,  which  has  now 
been  opened,  extends  from  the  pectoral  girdle  to  the  cloaca  and 
along  the  sides  of  the  latter;  it  communicates  with  the  exterior 
through  the  abdominal  pores  on  either  side  of  the  cloaca.  Pass  a 
bristle  or  probe  through  each  abdominal  pore  into  the  cloaca. 

Without  dissecting,  identify  the  following  parts  and  observe 
their  relations: 

The  peritoneum,  the  smooth  lining  of  the  body-wall,  which  is 
reflected  over  the  viscera. 

The  liver,  a  large,  gray  organ  attached  anteriorly  and  almost 
completely  divided  into  two  lobes  which  extend  well  back  along 
the  sides  of  the  abdominal  cavity. 

The  stomach,  lying  between  the  lobes  of  the  liver.  Its  posterior 
end  is  bent  forward  upon  itself  in  the  form  of  a  U.  The  two  limbs 
of  the  stomach  are  known  as  the  cardiac  (proximal)  and  pyloric 
(distal),  respectively.  With  a  second  turn  to  the  right  and  back- 
ward it  enters  the  intestine. 

The  intestine,  a  large,  thin-walled  tube  extending  from  the 
stomach  to  the  cloaca. 


6 

The  spleen,  a  dark,  triangular  mass  attached  to  the  posterior 
border  of  the  curve  of  the  stomach. 

The  spleen  of  Eugaleus  is  a  long,  slender  body  extending  from  the  middle 
of  the  proximal  limb  of  the  stomach  around  the  posterior  end  of  that  organ 
and  forward  again  along  the  distal  limb  for  two-thirds  of  the  length  of  the 
latter. 

The  pancreas,  a  firm  white  mass  the  larger  part  of  which  lies 
dorsal  to  the  posterior  end  of  the  stomach.  One  extremity  lies  on 
the  ventral  surface  of  the  junction  of  the  stomach  and  intestine. 

The  reproductive  glands,  (ovaries  or  testes),  lying  on  either  side 
of  the  midline  dorsal  to  the  anterior  portion  of  the  liver;  they  may 
be  mistaken  for  small  lobes  of  the  liver. 

The  reproductive  glands  of  Eugaleus  are  long  bodies  lying  above  the  stomach 
and  intestine.  They  are  fused  to  each  other  for  almost  their  entire  length. 

The  kidneys,  two  long,  slender,  brownish  bodies  extending  along 
the  dorsal  wall  of  the  abdominal  cavity  outside  the  peritoneum,  on 
either  side  of  the  midline. 

The  dogfish  usually  furnished  for  dissection  are  immature,  hav- 
ing the  genital  glands  and  ducts  only  partly  developed.  In  ma- 
ture females  the  oviducts  are  conspicuous  tubes  ventral  to  the  kid- 
neys. In  young  specimens  they  appear  as  slender,  white  tubes  ex- 
tending along  the  inner  borders  of  the  kidneys.  Anteriorly,  the 
oviducts  pass  ventrad  over  the  front  of  the  liver  to  the  ventral  wall 
of  the  body;  at  the  same  time  they  unite  to  form  a  funnel,  the 
ostium  tubae,  which  opens  into  the  coelom.  Vestigial  oviducts  open- 
ing into  the  coelom  are  found  in  the  same  position  in  males. 

In  males,  the  vasa  deferentia  appear  as  slender,  irregularly 
coiled  white  tubules  lying  near  the  medial  border  of  the  kidneys; 
they  are  much  less  conspicuous  than  the  oviducts,  especially  in 
young  males. 

THE  ALIMENTARY  SYSTEM.  In  dissecting  the  following  organs, 
care  should  be  taken  not  to  break  the  connections  of  the  organs  with 
each  other  or  with  other  parts,  or  to  cut  blood  vessels.  Organs 
should  not  be  removed  until  such  procedure  is  directed. 

The  mouth  and  pharynx  can  be  studied  to  better  advantage  later 
with  the  dissection  of  portions  of  the  vascular  system. 

The  oesophagus  can  be  seen  above  the  liver,  by  pressing  that 
organ  aside,  as  a  somewhat  constricted  tube  entering  the  anterior 
end  of  the  abdominal  cavity.  It  immediately  joins  the  stomach, 
which  is  more  or  less  expanded  according  to  the  amount  of  food 
contained  in  it. 

The  stomach  passes  directly  back  for  more  than  half  of  the 
length  of  the  abdominal  cavity,  then  turns  abruptly  forward,  form- 
ing a  distal  limb  about  a  third  as  long  as  the  proximal.  (Two- 
thirds  to  three-quarters  as  long  in  Eugaleus.)  The  distal  limb 
ends  with  a  sharp  turn  to  the  right,  where  it  is  constricted  by  the 
pyloric  sphincter,  which  marks  the  end  of  the  stomach. 

The  narrow  beginning  of  the  intestine  forming  the  turn  to  the 
right  and  backward  is  frequently  distinguished  as  the  duodenum. 
It  leads  from  the  stomach  directly  into  the  large  intestine,  a  wide, 
straight  tube  marked  externally  by  a  spiral  line  of  several  turns. 
The  large  intestine  narrows  posteriorly,  forming  a  region  some- 
what arbitrarily  termed  the  rectum,  which  opens  into  the  cloaca 
through  the  anus. 

Dorsal  to  the  rectum  and  attached  to  that  body  is  a  narrow 
spindle-shaped  body,  the  rectal  or  digitiform  gland. 

The  liver  is  attached  to  the  anterior  wall  by  a  broad  base,  the 
peritoneum  being  reflected  over  the  entire  remaining  surface.  The 


attaching  fold  of  the  peritoneum  is  frequently  called  the  suspensory 
ligament.  The  peritoneum,  or  coelomic  epithelium,  can  be  dis- 
sected easily  from  the  surface  of  the  liver  or  the  kidney  and  its 
extreme  thinness  and  delicacy  noted.  It  consists  of  a  single  layer 
of  cells. 

Most  of  the  abdominal  organs  are  suspended  from  the  dorsal 
wall  of  the  body  cavity  by  delicate  membraneous  sheets,  or  mesen- 
teries. Similar  sheets  between  the  organs  are  the  omenta.  The 
stomach  is  suspended  by  a  mesogaster,  which  extends  as  a  free  fold 
along  the  body  as  far  as  the  anterior  mesenteric  and  lienogastric 
arteries.  It  encloses  these,  and  is  attached  to  the  spleen,  pancreas, 
stomach,  and  anterior  end  of  the  intestine. 

The  spleen  is  connected  with  the  stomach  by  the  Castro-splenic 
omentum,  formed  by  an  extension  of  the  peritoneal  coat  of  the 
stomach  around  the  spleen.  The  liver  is  connected  to  the  loop  of 
the  stomach  by  the  gastro-hepatic  omentum  in  which  are  the  hepatic 
duct,  portal  vein,  and  hepatic  artery.  Near  the  stomach  it  is  joined 
by  a  fold  of  the  peritoneum  from  the  duodenum,  the  duodeno-hepatic 
omentum,  which  also  unites  with  the  mesogaster. 

The  rectum  and  rectal  gland  are  supported  by  a  second  median 
mesentary,  the  mesorectum. 

In  Eugaleus  the  mesentery  extends  the  entire  length  of  the  abdominal 
cavity.  It  forms  a  broad  sheet  attached  to  the  anterior  end  of  the  proximal 
limb  of  the  stomach  (mesogaster),  to  the  anterior  end  of  the  intestine  (mesen- 
tery proper),  and  to  the  rectum  (mesorectum).  There  is  not  the  reduction  of 
the  mesentery  which  there  is  in  Squalus.  The  gonads  are  suspended  from  the 
lateral  faces  of  the  mesentery  above  the  stomach  and  intestine.  The  gastro- 
hepatic  omentum  forms  a  broad  sheet  between  the  limbs  of  the  stomach,  join- 
ing the  mesogaster  dorsal  to  the  stomach  and  the  mesentery  above  the  intestine. 

A  small  division  of  the  right  lobe  of  the  liver  stands  out  be- 
tween the  main  lobes.  In  this  is  located  a  long,  narrow  gall-bladder. 
Open  the  bladder  by  a  longitudinal  ventral  incision.  The  opening 
into  the  bile-duct  will  be  found  near  the  anterior  end  of  the  bladder. 

In  Eugaleus,  which  does  not  possess  such  a  median  lobe,  the  gall-bladder 
lies  hidden  in  the  right  lobe  of  the  liver.  It  can  be  opened  and  explored,  but 
the  connection  with  the  duct  can  usually  be  demonstrated  only  by  scraping. 
Do  this  later. 

The  bile-duct  passes  along  the  dorsal  side  of  the  gall-bladder 
and  the  edge  of  the  gastro-hepatic  and  duodeno-hepatic  omenta  to 
the  junction  of  the  duodenum  and  large  intestine,  where  it  opens 
into  the  alimentary  canal.  Trace  its  oblique  course  through  the  wall 
of  the  intestine.  The  bile  duct  and  the  collecting  (hepatic)  ducts 
of  the  liver  will  be  traced  in  the  liver  at  a  later  stage  of  the  dissec- 
tion. 

The  pancreas  consists  of  two  lobes;  a  slender  lobe  lying  dorsal 
to  and  parallel  with  the  stomach,  and  a  flattened  oval  lobe  lying 
upon  the  ventral  surface  of  the  duodenum,  connected  with  the  dorsal 
lobe  by  a  slender  bar  of  glandular  tissue. 

The  pancreatic  duct  passes  from  the  extreme  right  end  of  the 
duodenal  lobe  obliquely  through  the  wall  of  the  intestine,  opening 
into  the  anterior  end  of  the  large  intestine.  Free  the  edge  of  the 
lobe  from  the  peritoneum  and  follow  the  duct. 

Open  the  proximal  limb  of  the  stomach  by  a  ventral  incision 
which  shall  not  cut  any  large  blood  vessels.  Wash  out  the  in- 
terior. Observe  the  three  coats  of  the  stomach;  the  outer  peritoneal, 
the  middle  muscular,  and  the  inner  mucous  coats.  In  the  anterior 
portion  of  the  stomach  the  mucous  coat  projects  in  the  form  of  large 
papillae  (absent  in  Eugaleus).  Posterior  to  these,  observe  the  irreg- 
ular folding  of  the  mucous  coat,  depending  upon  the  degree  of  con- 
traction of  the  muscular  coat. 


8 

The  muscular  coat  consists  of  an  outer  circular  and  an  inner 
longitudinal  layer  of  muscle  fibres.  Separate  the  two  layers  from 
each  other  and  from  the  mucous  coat;  observe  the  network  of 
blood  vessels  between  the  longitudinal  muscles  and  the  mucosa. 

Open  the  pyloric  end  of  the  stomach,  continuing  the  cut  through 
the  pylorus  into  the  intestine.  Examine  the  coats  as  before,  observ- 
ing especially  that  an  outer  layer  of  longitudinal  muscle  fibres  is 
frequently  developed,  and  that  the  pyloric  valve  is  formed  by  an 
increase  in  the  thickness  of  the  coat  of  circular  fibres. 

Cut  through  the  wall  of  the  large  intestine  along  the  right  side 
from  its  anterior  end  to  the  rectum.  Do  not  cut  deeper  than  the 
thickness  of  the  wall.  Corresponding  to  the  external  markings,  the 
mucous  membrane  projects  internally  in  a  spiral  fold,  known  as 
the  spiral  valve.  Separate  the  wall  of  the  intestine  from  the  edge 
of  the  spiral  fold  upon  both  sides  of  the  longitudinal  incision,  ex- 
posing a  considerable  surface  of  the  valve.  Wash  well,  and  ob- 
serve the  character  of  the  valve,  the  direction  of  the  folds,  and  the 
manner  of  the  reversal  of  their  direction  which  usually  takes  place 
in  the  posterior  half  of  the  valve. 

Cut  across  the  rectal  gland  at  its  middle.  Observe  the  character 
of  its  tissues,  and  then  insert  a  bristle  into  the  central  cavity  of  the 
gland  and  pass  it  into  the  rectum.  Open  the  rectum  and  note  the 
point  of  communication  of  the  two  organs. 

URINARY  AND  REPRODUCTIVE  ORGANS.  The  kidneys  (mesonephri, 
Wolff ian  bodies],  are  slender  bodies  extending  along  the  entire 
length  of  the  dorsal  wall  of  the  abdomen.  The  posterior  moiety  of 
each  is  thicker  and  wider  than  the  anterior,  which  appears  to  have 
largely  lost  the  functions  of  excretion  in  adult  dogfish.  Notice  the 
position  of  the  kidneys  outside  the  peritoneum. 

THE  MALE.  The  tester  are  white  bodies  lying  to  the  right  and 
left  of  the  oesophagus,  dorsal  to  the  anterior  portion  of  the  liver. 
Each  is  suspended  by  a  fold  of  the  peritoneum,  the  mesorchium. 
(The  testes  of  Galeus  are  long  bodies  attached  to  the  sides  of  the 
mesentery.) 

Showing  through  the  peritoneum,  a  much  convoluted,  white  tube 
can  be  seen  on  the  ventral  surface  of  the  kidney.  This  is  the 
mesonephric  or  Wolffian  duct.  In  young  specimens  it  may  be 
nearly  straight,  lying  near  the  medial  border  of  the  kidney.  In  adult 
specimens  it  can  be  followed  forward  as  far  as  the  anterior  end  of 
the  testis.  While  the  Wolffian  duct  is  the  duct  of  the  kidney,  and  is 
joined  by  tubules  of  the  anterior  part  of  the  kidney,  it  is  so  modified 
in  the  male  that  its  principal  function  is  to  serve  as  the  duct  of  the 
testis,  a  vas  deferens.  The  collecting  tubules  of  the  posterior  part 
of  the  kidney  join  to  form  a  urinary  duct  which  is  independent  of 
the  Wolffian  duct.  The  posterior  end  of  the  Wolffian  duct  is  straight 
and  considerably  expanded,  forming  a  large  seminal  vesicle.  The 
duct  becomes  more  and  more  closely  convoluted  as  it  passes  for- 
ward, and  the  kidney  tissue  overlying  it  diminishes.  At  the  an- 
terior end  of  the  mesonephros  the  Wolffian  duct  forms  a  mass  of 
tubules,  the  epididymis.  Very  small  tubules,  the  vasa  efferentia, 
pass  from  the  anterior  end  of  the  testis  to  the  epididymis.  These 
are  difficult  for  the  student  to  distinguish. 

Cut  through  the  peritoneum  along  the  outer  side  of  one  kidney. 
Then  strip  the  peritoneum  toward  the  inner  border  of  the  kidney. 
The  urinary  duct  will  usually  be  closely  attached  to  the  peritoneum 
and  parallel  with  the  Wolffian  duct,  but  nearer  the  midline  of  the 
body.  The  urinary  duct  can  be  separated  from  the  peritoneum  by 


9 

a  little  careful  work.  Numerous  small  ducts  pass  from  the  kidney 
into  the  urinary  duct. 

Open  the  uro-genital  papilla  near  its  tip  and  extend  the  incision 
forward  so  as  to  open  the  sac  connected  with  the  base  of  the  papilla. 
The  pore  at  the  tip  of  the  papilla  leads  into  a  space  within  the 
papilla  itself,  the  uro-genital  sinus,  which  branches  to  the  left  and 
right  in  pouches  which  extend  beyond  the  posterior  ends  of  the  vasa 
deferentia.  These  cornua  of  the  uro-genital  sinus  are  of  variable 
length,  and  are  often  named  sperm-sacs.  In  a  mature  male  they 
may  be  found  to  be  filled  with  sperm,  as  may  also  the  seminal 
vesicles  and  the  convoluted  portion  of  the  Wolffian  duct.  The 
openings  of  the  vasa  deferentia  into  the  sinus  are  large  and  easily 
located.  The  urinary  duct  opens  into  the  sinus  by  a  separate  pore 
just  behind  the  opening  of  the  vas  deferens. 

Cut  open  the  seminal  vesicle  and  part  of  the  convoluted  vas 
deferens.  The  space  within  is  subdivided  by  transverse  folds  or 
lamellae  extending  from  a  longitudinal  ridge. 

No  vasa  efferentia  can  be  distinguished  in  Eugaleus.  The  anterior  extremi- 
ties of  the  kidney  and  testis  of  each  side  come  into  close  contact  with  each 
other  and  here  the  vasa  efferentia  pass  from  the  testis  to  the  vas  deferens.  The 
sperm-sac  is  a  large  blind  pouch,  one  or  two  inches  in  length,  leading  out  of 
the  posterior  end  of  the  vas  deferens,  and  directed  forward  along  its  side.  The 
vas  deferens  of  Eugaleus  is  not  convoluted. 

In  the  young  specimens  usually  supplied  to  laboratories  the  vas 
deferens  is  straight  and  no  seminal  vesicle  is  developed.  The  vasa 
efferentia  are  more  difficult  to  see;  otherwise  the  relations  of  the 
urinary  and  genital  organs  are  as  in  the  adult. 

The  suspensory  ligament  of  the  liver  is  continued  posteriorly 
along  the  midline  of  the  ventral  body-wall;  the  dorsal  edge  sup- 
ports a  funnel  which  opens  into  the  abdominal  cavity  by  a  long, 
narrow  mouth.  From  the  anterior  end  of  the  funnel  two  narrow 
tubes  pass  to  the  right  and  left  over  the  anterior  surface  of  the 
liver.  They  end  blindly  in  the  tissues  dorsal  to  the  anterior  end  of 
the  liver.  These  are  vestiges  of  the  Muellerian  ducts  (pronephric 
ducts)  which  form  the  oviducts  of  the  females. 

THE  FEMALE.  The  ovaries  are  large,  white  bodies  lying  at  the 
sides  of  the  stomach,  dorsal  to  the  lobes  of  the  liver.  Each  is  cov- 
ered by  the  peritoneum  and  suspended  by  a  fold  of  the  same,  the 
mesovarium.  Ova  of  various  sizes  may  be  felt  in  the  tissue  of  the 
ovary,  which  should  be  exposed  by  dissection. 

The  ovaries  of  Eugaleus  are  long  slender  bodies  lying  on  either  side  of 
the  mesogaster,  dorsal  to  the  stomach  and  intestine.  Their  posterior  portions 
are  fused. 

The  oviducts  (Muellerian  ducts)  are  large  tubes  suspended  from 
between  the  kidneys  by  a  narrow  peritoneal  band.  The  posterior 
portion  of  the  oviduct,  where  development  of  the  eggs  takes  place, 
is  considerably  enlarged.  Each  oviduct  opens  separately  into  the 
cloaca  by  a  pore  at  the  side  of  the  urinary  papilla.  Followed  for- 
ward, the  oviducts  pass  over  the  anterior  surface  of  the  liver  and 
following  a  continuation  of  the  suspensory  ligament,  bend  around 
posteriorly  and  unite.  At  the  point  of  union  they  open  into  the 
coelom  by  a  common,  large,  funnel-shaped  aperture,  the  ostium 
tubae. 

Cut  through  the  peritoneum  along  the  outer  side  of  one  kidney. 
Then  strip  the  peritoneum  toward  the  inner  side  of  the  kidney. 
Numerous  small  excretory  ducts  will  be  seen  joining  the  main 
urinary  duct  (Wolffian  duct,  mesonephric  duct),  which  runs  along 
the  inner  margin  of  the  kidney.  Make  an  incision  in  the  side  of 


10 

the  urinary  papilla  to  open  the  cavity  within  it,  the  urinary  sinus. 
The  connection  of  this  with  the  pore  at  the  tip  of  the  papilla  should 
be  demonstrated.  Extend  the  incision  forward.  The  urinary  sinus 
divides  into  right  and  left  cornua  which  are  of  considerable  size  and 
lie  dorsal  to  the  oviducts.  Trace  the  Wolffian  duct  to  the  urinary 
sinus  and  demonstrate  its  opening  into  the  cornu  anterior  to  the 
point  where  the  two  cornua  unite. 

In  young  specimens  the  ovaries  are  small,  and  the  oviducts  are 
narrow,  white  tubes  lying  along  the  medial  margins  of  the  kidneys. 

Nephrostomes,  short,  segmen tally  arranged  kidney  tubules  which 
open  to  the  coelom  by  a  funicular  aperture,  are  found  by  a  close 
examination  along  the  medial  border  of  each  kidney.  They  should 
be  observed  carefully  with  the  aid  of  a  good  dissecting  lens. 
Learn  the  significance  of  these  structures. 

In  the  course  of  development  two  sets  of  nephridia  (kidneys) 
are  formed.  The  first  (pronephros)  develops  just  back  of  the  head 
of  the  embryo,  but  does  not  persist  in  the  adult.  Its  duct,  known 
commonly  as  the  Muellerian  duct,  develops  into  the  functional  ovi- 
duct of  the  female,  but  forms  an  apparently  useless  vestige  in  the 
male.  The  second  kidney  (mesonephros)  develops  behind  the  first 
and  is  the  excretory  organ  of  the  adult.  Its  duct  (frequently  given 
the  name  of  Wolffian  duct)  is  the  urinary  duct  in  the  female,  but 
functions  in  the  male  chiefly  as  a  sperm  duct,  and  therefore  is 
called  the  vas  deferens.  The  collecting  tubules  of  the  posterior 
portion  of  the  kidney  of  the  male  unite  to  form  a  urinary  duct  which 
opens  into  the  Wolffian  duct  or  the  urogenital  sinus. 

RESPIRATORY  ORGANS 

Open  the  anterior  gill-pouch  of  the  left  side  by  dorsal  and  ven- 
tral cuts  extending  from  the  angles  of  the  cleft,  but  cutting  only  as 
far  as  is  necessary  to  see  the  structures  within  the  pouch.  Upon  the 
medial  side  the  gill  pouch  opens  into  the  pharynx  by  a  dorso-ventral 
slit,  guarded  by  projecting  cartilagineous  gill-rakers,  which  pre- 
vent particles  of  food  from  passing  into  the  gill  pouch  with  the 
respiratory  current.  On  both  the  anterior  and  posterior  wall  of  the 
pouch  is  a  demi  branch.  If  the  specimen  is  injected  a  large  blood 
vessel  can  be  seen  through  the  skin  in  the  inner  border  of  the  demi- 
branch,  and  small  vessels  passing  from  this  into  the  leaflets,  where 
the  interchange  of  gases  between  the  water  and  blood  takes  place. 

Open  similarly  each  pouch  of  the  same  side,  observing  the  num- 
ber of  the  demi-branchs  and  their  relation  to  the  pouches. 

Upon  the  anterior  wall  of  the  spiracle  demonstrate  a  row  of 
small  vestigial  gills;  being  supplied  with  arterial  instead  of  venous 
blood  they  form  what  is  termed  a  pseudobranch. 

VASCULAR  SYSTEM 

HEART  AND  VENTRAL  AORTA.  Continue  the  longitudinal  incision 
through  the  skin  as  far  as  the  mandible.*  Dissect  away  the  sheet 
of  muscles  between  the  gill-pouches  and  the  mandible,  exposing  a 
slender  muscle  which  extends  from  the  pectoral  girdle  to  the  middle 
of  the  mandible.  The  thyroid  gland  lies  dorsal  to  the  anterior  end 
of  this  muscle,  close  against  the  mandible.  (The  thyroid  of  Euga- 
leus  is  a  broad,  flattened  structure  covering  the  anterior  ends  of  the 
coracohyoideus  muscles.)  Carefully  dissect  out  the  muscles  lying 
between  the  branchial  pouches  of  the  right  and  left  sides. 

*See  footnote,  p.  31. 


11 

In  front  of  the  pectoral  girdle  lies  a  thin  walled  sac,  the  peri- 
cardial  sac.  Open  it  by  a  median  ventral  incision.  Remove  about 
one-half  inch  of  the  middle  of  the  pectoral  girdle,  being  careful  not 
to  cut  the  thin-walled  part  of  the  heart  lying  dorsal  to  it.  The 
pericardial  cavity  is  a  pear-shaped  chamber  containing  the  heart, 
and  lined  by  the  smooth  pericardium  which  is  morphologically 
equivalent  to  the  peritoneum.  At  the  anterior  extremity  of  the 
chamber  the  pericardium  is  seen  to  be  reflected  backward  over  the 
surface  of  the  heart,  thus  forming  its  smooth  outer  coat. 

The  heart  may  be  considered  as  a  bent  tube,  enlarged  in  certain 
regions  to  form  the  chambers.  Anteriorly  and  ventrally  is  a  short, 
thick-walled  tube,  the  conus  arteriosus;  this  leads  out  of  the  peri- 
cardial sac  anteriorly,  while  posteriorly  it  opens  into  a  large  mus- 
cular chamber,  the  ventricle.  Dorsal  to  the  ventricle,  and  project- 
ing on  either  side  of  it  is  the  thin  walled  auricle.  Dorsal  to  both 
ventricle  and  auricle  is  the  extremely  thin-walled  sinus  venosus.  This 
is  triangular  in  shape,  the  apex  opening  into  the  posterior  side  of 
the  auricle,  the  base  attached  to  the  posterior  wall  of  the  pericardial 
cavity;  the  lateral  angles  are  drawn  out  into  the  ducti  cuvierii, 
which  receive  veins  from  the  anterior  and  posterior  parts  of  the 
body.  From  the  conus  arteriosus  springs  a  smaller  vessel,  the 
ventral  aorta,  which  passes  forward  between  the  gill  pouches.  Take 
note  of  the  small  arteries  passing  over  the  surface  of  the  conus  and 
along  the  inner  ends  of  the  gill  pouches,  and  take  care  not  to  cut 
them  or  their  branches  in  the  subsequent  dissection. 

Two  pairs  of  arteries  leave  the  ventral  aorta  as  it  emerges  from 
the  pericardial  sac.  The  aorta  then  passes  forward  some  distance 
and  finally  divides  into  two  branches  which  pass  to  either  side.  Fol- 
low the  branches  of  the  aorta  outward  on  the  left  side  and  demon- 
strate their  courses.  The  anterior  branch  quickly  divides  into  two, 
the  anterior  of  these  passing  along  the  base  of  the  first  demibranch. 
The  posterior  enters  the  septum  between  the  first  and  second  pouches, 
and  supplies  the  second  and  third  demibranchs.  The  middle  branch 
of  the  aorta  passes  directly  to  the  fourth  and  fifth  demibranchs.  The 
posterior  branch  divides  almost  as  it  leaves  the  aorta,  its  branches 
supplying  the  remaining  demibranchs.  There  is  considerable  varia- 
tion in  this  branch  of  the  aorta.  It  usually  divides  as  stated,  but  it 
frequently  passes  some  distance  toward  the  gills  before  dividing, 
and  in  a  considerable  number  of  cases  two  vessels  arise  directly 
from  the  aorta  instead  of  one. 

The  arteries  carrying  blood  from  the  ventral  aorta  to  the  gills 
are  named  the  afferent  branchial  arteries.  Observe  the  relation  of 
these  vessels  to  the  gills. 

VENOUS  SYSTEM.  All  the  blood  of  the  body  is  conveyed  to  the 
sinus  venosus.  The  sides  of  the  sinus  venosus  are  extended  as  large 
vessels,  already  referred  to  as  the  ducti  cuvierii.  Open  the  sinus 
and  ducti  by  a  transverse  ventral  incision.  The  ducti  pass  directly 
into  the  lateral  veins.  Near  the  middle  of  the  posterior  wall  of  the 
sinus  is  an  aperture  of  varying  size,  the  opening  of  the  hepatic 
sinus;  there  are  rarely  two  openings  in  Squalus,  always  two  in 
Eugaleus.  A  large  opening  on  the  posterior  wall  of  each  ductus 
leads  into  the  posterior  cardinal  vein.  On  the  anterior  wall  of  the 
ductus,  near  the  sinus  venosus,  is  a  small  aperture,  that  of  the  in- 
ferior jugular  vein.  Lateral  to  this  is  frequently  a  somewhat  larger 
opening  of  the  anterior  cardinal  vein.  This  is  absent,  however,  in 


12 

the  majority  of  specimens;  the  anterior  cardinals  opening  into  the 
anterior  ends  of  the  posterior  cardinals  in  about  six  out  of  ten  cases. 

A  large  cavity,  the  hepatic  sinus,  exists  in  the  anterior  end  of  the 
liver  just  posterior  to  the  suspensory  ligament.  Cut  into  the  liver 
at  this  point  until  the  sinus  is  found,  open  it,  and  observe  the  large 
hepatic  veins  bringing  blood  into  it  from  the  liver,  as  well  as  its 
communication  with  the  sinus  venosus. 

Trace  all  veins  by  passing  a  flexible  probe  or  guarded  bristle 
along  them  and  then  opening  the  vein  with  the  probe  as  a  guide. 
All  smaller  vessels  emptying  into  those  described  should  be  noted. 

The  lateral  veins  pass  forward  to  the  posterior  edge  of  the  pec- 
toral girdle,  bend  sharply  dorsad,  and  enter  the  lateral  extremities 
of  the  ducti  cuvierii.  Open  a  lateral  vein  near  the  anterior  end  and 
trace  it  toward  the  heart.  The  right  and  left  lateral  veins  are  joined 
by  a  vein  passing  along  the  ventral  bar  of  the  pectoral  girdle.  Open 
the  lateral  veins  at  a  point  about  two  inches  in  front  of  the  pelvic 
girdle  and  trace  the  veins  backward  as  far  as  they  can  be  followed. 
The  blood  from  the  pelvic  fins  enters  the  lateral  vein  through  the 
femoral  vein.  The  lateral  veins  finally  unite  back  of  the  cloaca. 

Just  before  the  lateral  vein  enters  the  ductus  cuvierius  it  is  joined 
by  a  large  coracoid  vein  which  runs  dorsad  and  posteriorly  along 
the  posterior  edge  of  the  pectoral  arch.  Follow  its  course.  It  re- 
ceives a  good-sized  pectoral  vein  from  the  pectoral  fin,  and  some- 
times several  smaller  veins  from  the  same  region.  Traced  dorsad  it 
is  found  to  open  into  a  large  blood  sinus  above  the  liver  and  oesoph- 
agus, the  cardinal  sinus. 

In  Eugaleus  this  connecting  vein  between  the  lateral  vein  and  the  cardinal 
sinus  is  wanting,  the  pectoral  vein  opening  directly  into  the  lateral. 

The  ventral  cutaneous  vein,  which  runs  along  the  ventral  mid- 
line  of  the  body  wall,  should  be  followed;  anteriorly  it  joins  the 
vessel  uniting  the  two  laterals;  posteriorly  it  divides  at  the  pelvic 
arch  and  anastomoses  with  the  laterals. 

Pass  a  bristle  from  the  sinus  venosus  into  one  of  the  posterior 
cardinal  veins  and  trace  the  vein  backward  between  the  kidneys  as 
far  as  possible.  Open  both  posterior  cardinals  in  this  way,  washing 
them  out  and  observing  that  they  receive  blood  from  the  kidneys  by 
a  series  of  renal  veins,  and  that  they  are  separate  in  their  posterior 
parts,  but  communicate  with  each  other  anteriorly,  where  they  are 
greatly  expanded;  the  communicating  portions  and  coincident  en- 
largement forming  the  cardinal  sinus.  The  anterior  portion  of  the 
cardinal  vein  receives  ovarian  or  spermatic  veins  from  the  female 
or  male  gonad,  anterior  oviducal  veins  from  the  anterior  part  of 
the  oviduct,  and  segmental  veins  from  the  corresponding  region  of 
the  body  wall.  There  sometimes  is  more  than  a  single  opening  from 
the  posterior  cardinal  vein  into  the  cuvierian  duct. 

Cut  across  the  tail  an  inch  behind  the  cloaca.  Two  vessels  lie 
in  the  cartilaginous  arch  below  the  centra  of  the  vertebrae;  the 
dorsal  of  the  two  is  the  caudal  artery,  the  ventral  one  is  the  caudal 
vein.  Follow  the  vein  forward.  Dorsal  to  the  cloaca  it  divides  into 
two,  which  should  be  followed  along  the  dorsal  surfaces  of  the  kid- 
neys. These  are  the  renal  portal  veins,  conveying  blood  to  the  kid- 
neys. Besides  collecting  the  blood  of  the  tail  the  renal  portals  also 
receive  the  posterior  oviducal  and  segmental  veins.  They  pass  into 
the  capillaries  of  the  kidneys. 

The  inferior  jugular  vein  opens  into  the  medial  end  of  the  cuvier- 
ian duct.  Trace  it  forward  along  the  ventral  ends  of  the  gill-pouches; 
it  receives  vessels  from  the  arches  and  finally  joins  the  hyoidean 


13 

veins  which  follow  the  hyoid  arch.  At  the  outer  end  of  the  cuvierian 
duct  there  is  often  a  small  opening  on  the  anterior  wall  opposite  the 
mouth  of  the  posterior  cardinal  vein.  This  leads  into  the  anterior 
cardinal  vein.  As  mentioned  before,  in  a  slight  majority  of  the 
cases  examined,  the  anterior  cardinal  vein  opens  into  the  posterior 
cardinal  vein,  not  directly  into  the  cuvierian  duct.  If  possible,  pass 
a  bristle  into  the  anterior  cardinal.  To  follow  the  vein,  and  usually 
this  is  the  best  way  to  find  it,  make  a  vertical  longitudinal  incision 
upon  the  dorsal  side  of  the  neck,  between  the  gill  pouches  and  the 
mass  of  muscle  lying  beside  the  vertebral  column.  This  will  open 
the  anterior  cardinal,  which  is  considerably  expanded  in  this  region, 
and  it  may  be  traced  from  this  point  toward  the  heart  and  the  head. 
The  anterior  cardinal  narrows  suddenly  in  front  of  the  anterior  gill 
pouch,  and  leads  downward  to  the  orbit,  where  it  expands  into  the 
orbital  sinus  surrounding  the  eyeball  and  its  muscles.  Trace  the 
anterior  cardinal  only  as  far  as  the  opening  into  the  orbital  sinus  at 
this  time.  Veins  from  the  anterior  portion  of  the  head  and  from 
the  brain  can  be  followed  when  the  dissection  of  the  eye  is  under- 
taken. 

Just  back  of  the  spiracle  the  anterior  cardinal  receives  the  hyoi- 
dean  vein.,  which  passes  ventrad  along  the  base  of  the  first  demi- 
branch  and  unites  with  the  hyoidean  of  the  opposite  side.  Ventrally, 
it  also  communicates  with  the  inferior  jugular  vein. 

The  principal  veins  of  the  body  have  now  been  dissected  with 
the  exception  of  the  hepatic  portal  vein,  which  it  is  better  to  trace 
after  the  arteries  of  the  digestive  tract  have  been  studied. 

THE  EFFERENT  BRANCHIAL  ARTERIES  AND  DORSAL  AORTA.  Com- 
mencing at  the  mouth,  cut  through  the  floor  of  the  pharynx  close  to 
the  left  side  of  the  ventral  aorta  and  the  heart.  The  cut  should  leave 
the  gill  arches  uninjured,  and  may  be  continued  into  the  oesophagus. 

Examine  the  interior  of  the  mouth  and  pharynx,  observing  par- 
ticularly the  form  and  arrangement  of  the  teeth,  the  spiracular  and 
branchial  clefts,  the  gill-rakers,  and  the  character  of  the  mucous 
coat  of  the  pharynx. 

Remove  the  skin  from  the  roof  of  the  pharynx.  This  exposes 
four  pairs  of  efferent  branchial  arteries  bringing  blood  from  the 
gills  and  uniting  in  pairs  to  form  the  dorsal  aorta.  Follow  each  ves- 
sel of  the  left  side  out  to  its  gill-cleft.  At  the  dorsal  end  of  the  gill- 
cleft  it  divides  into  a  large  posterior  and  small  anterior  branch. 
These  respectively  pass  along  the  posterior  and  anterior  demibranchs 
of  the  gill  pouch,  receiving  fine  branches  from  the  gill  lamellae, 
and  finally  unite  again  at  the  ventral  end  of  the  gill-pouch.  Thus 
a  complete  loop  is  formed  around  the  branchial  cleft.  The  posterior 
branch  of  each  efferent  artery  and  the  anterior  branch  of  the  suc- 
ceeding one  are  united  by  several  short  vessels.  The  efferent  artery 
of  the  last  demibranch  possesses  only  these  connections  with  the 
branch  next  anterior  to  it,  and  none  with  the  aorta  directly.  From 
the  ventral  ends  of  the  efferent  loops  small  vessels  pass  toward  the 
mid-line  to  unite  with  a  longitudinal  artery,  the  hypobranchial  ar- 
tery, which  will  be  traced  farther  a  little  later  in  the  dissection. 

In  Eugaleus  the  dorsal  aorta  extends  forward  beyond  the  union  of  the  first 
pair  of  efferent  branchials  and  then  divides  into  small  right  and  left  branches 
which  pass  forward  and  outward  to  unite  with  the  common  carotid  arteries. 

A  common  carotid  artery  leaves  the  dorsal  end  of  each  anterior 
efferent  branchial  loop,  passing  forward  and  inward.  At  the  level 
of  the  spiracles  it  divides  into  external  and  internal  carotids;  the 
internal  carotid  unites  with  its  fellow  of  the  opposite  side  and  enters 
the  skull.  The  external  carotid  arteries  run  outward  and  forward 


14 

around  the  eyes  and  are  distributed  to  the  regions  of  the  mandible 
and  snout.  Do  not,  at  present,  trace  them  beyond  the  posterior 
edge  of  the  eye. 

Another  vessel  arises  from  the  middle  of  the  anterior  side-  of 
the  first  efferent  branchial  loop  and  runs  forward  to  the  spiracle, 
where  it  ends  in  the  capillaries  of  the  pseudobranch.  This  is  the 
afferent  hyoidean  artery.  The  term  pseudobranch  is  used  for  the 
branchial  lamellae  of  the  spiracle  rather  than  demibranch  because 
of  the  arterial  blood  supply  of  this  organ. 

Immediately  after  uniting  the  internal  carotids  divide  and  di- 
verge, forming  an  X-shaped  figure.  Each  anterior  limb  of  the  X 
again  divides  into  two  branches.  The  lateral  branch  passes  to  the 
ventral  surface  of  the  skull;  it  presently  gives  off  an  anterior  twig 
(ophthalmic  artery]  which  enters  the  eye.  It  then  passes  on  as  the 
efferent  hyoidean  artery  to  the  pseudobranch.  The  inner  of  the  two 
branchs  mentioned  above  passes  on  as  the  internal  carotid,  sensu 
strictu,  and  is  distributed  to  the  brain. 

If  the  dissection  is  made  with  care,  the  branches  of  the  internal 
carotid  can  all  be  found  without  cutting  any  important  nerves.  The 
branches  passing  to  the  eye  and  brain  are  best  traced  to  their  ter- 
minations in  connection  with  the  dissection  of  the  nervous  system. 

Near  the  union  of  the  first  pair  of  efferent  branchial  arteries  a 
small  posterior  vertebral  artery  arises  from  each,  and  runs  anter- 
iorly along  the  vertebral  column. 

Near  the  divisions  of  the  common  carotids  two  anterior  vertebral 
arteries  arise  from  these  vessels  and  pass  posteriorly,  often  anasto- 
mosing with  the  posterior  vertebral  arteries.  These  vertebral  arteries 
are  vestiges  of  the  former  anterior  part  of  the  dorsal  aorta  (compare 
with  Eugaleus,  in  which  the  dorsal  aorta  sends  forward  two  vessels 
which  join  the  common  carotids). 

An  oesophageal  artery  springs  from  the  second  efferent  branch- 
ial, and  passes  back  until  it  enters  the  wall  of  the  oesophagus.  It 
also  gives  off  nutrient  branches  to  the  second,  third,  and  fourth 
gill  pouches.  The  nutrient  artery  of  the  first  gill  pouch  arises  di- 
rectly from  the  first  efferent  branchial. 

Near  the  point  at  which  the  fourth  pair  of  efferent  branchials 
join  the  aorta,  two  small  subclavian  arteries  leave  the  aorta  and 
pass  into  the  pectoral  fins.  There  is  some  variation  in  regard  to 
the  point  of  origin  of  these  vessels;  it  may  be  either  in  front  of  or 
behind  the  junction  of  the  fourth  efferent  branchials.  with  the  aorta. 

The  hypobranchial  artery  passes  along  the  ventral  ends  of  the 
gill  pouches.  It  is  either  connected  with  the  efferent  branchial  loops 
by  short  branches,  or  is  formed,  in  part  at  least,  by  short  vessels 
connecting  these  loops.  The  hypobranchials  are  important  nutrient 
vessels,  supplying  the  gill  pouches  and  the  muscles  of  the  throat 
and  the  oesophagus  by  means  of  numerous  small  arteries;  from  the 
hypobranchials  also  arise  small  posterior  coronary  arteries  which 
pass  to  the  ventral  and  posterior  walls  of  the  pericardium  and  the 
sinus  venosus,  and  larger  anterior  coronary  arteries  supplying  the 
ventricle  and  conus  arteriosus.  The  hypobranchials  can  frequently 
be  followed  along  the  dorsal  side  of  the  pericardium  and  then  out- 
ward to  junctions  with  the  subclavian  arteries. 

The  coeliac  artery  (coeliac  axis)  arises  from  the  aorta  just  back 
of  the  subclavians.  Passing  posteriorly  and  ventrad  close  to  the 
right  side  of  the  stomach  and  reaching  the  gastro-hepatic  omentum, 
it  divides  into  two  branches,  the  gastro-hepatic  and  anterior  intes- 
tinal arteries.  The  first  gives  off  a  small  hepatic  artery  to  the  liver 


15 

and  a  large  gastric  artery  to  the  cardiac  limb  of  the  stomach.  The 
anterior  intestinal  artery  supplies  the  pyloric  limb  of  the  stomach, 
the  pancreas,  duodenum,  and  right  side  of  the  large  intestine. 

Small  genital  arteries,  supplying  the  reproductive  glands,  arise 
from  the  coeliac  near  its  origin.  (In  Eugaleus  the  genital  arteries 
arise  from  the  anterior  and  posterior  mesenteric  arteries.) 

At  about  the  middle  of  the  abdominal  cavity  two  arteries  arise 
close  together  from  the  aorta.  The  anterior  of  the  two  is  the  an- 
terior mesenteric  artery;  it  passes  to  the  left  side  of  the  large  intes- 
tine and  its  branches  anastomose  more  or  less  with  those  of  the 
anterior  intestinal  artery.  The  posterior  vessel  is  the  lieno-gastric ; 
it  goes  to  the  spleen,  pancreas,  and  loop  of  the  stomach. 

The  posterior  mesenteric  artery  leaves  the  aorta  a  little  distance 
back  of  the  lienogastric  and  passes  to  the  rectal  gland,  rectum,  and 
cloaca. 

Free  the  kidney  from  the  body  wall  along  its  outer  edge  and 
turn  it  up  so  as  to  expose  its  dorsal  surface.  Observe  the  numerous 
parietal  arteries  (going  to  the  body  wall)  and  renal  arteries  (to  the 
kidney),  which  spring  from  the  dorsal  aorta.  Branches  of  the 
parietals  also  pass  into  the  kidney. 

A  pair  of  small  iliac  arteries  pass  into  the  pelvic  fins. 

Oviducal  arteries,  one  or  several  on  each  side,  arise  from  the 
aorta  behind  the  coeliac  artery  and  pass  to  the  oviduct.  Their  size 
varies  largely  with  the  development  and  physiological  condition  of 
the  oviduct. 

The  aorta  is  continued  in  the  tail  as  the  caudal  artery. 

DISSECTION  OF  THE  HEART.  Remove  the  heart  together  with  the 
ventral  aorta  from  the  body  and  fasten  it,  dorsal  side  up,  under 
water.  Open  the  sinus  venosus  with  scissors,  wash  it  out,  and  ob- 
serve the  vertical  slit-like  opening  into  the  auricle  and  the  two  mem- 
braneous valves  which  guard  it. 

Continue  the  cut  through  the  sinu-auricular  aperture  along  the 
median  dorsal  line  of  the  auricle;  observe  the  thin  walls  of  the 
auricle  and  their  strengthening  by  an  irregular  mesh  of  muscles, 
the  musculi  pectinati;  the  shape  and  position  of  the  auriculo-ventri- 
cular  aperture;  the  flaps  of  the  auriculo-ventricular  valve.  Press 
upon  the  sides  of  the  ventricle  and,  if  possible,  observe  the  mode  of 
action  of  the  valve. 

Cut  across  the  ventricle  from  the  auriculo-ventricular  aperture. 
Carry  another  incision  from  this  along  the  dorsal  side  of  the  conus 
arteriosus.  Observe  the  small  size  of  the  cavity  of  the  ventricle,  the 
thickness  of  its  walls,  and  the  projecting  network  of  muscles,  the 
columnae  carneae,  some  of  which  are  attached  to  the  edges  of  the 
auriculo-ventricular  valves. 

In  the  conus  arteriosus  observe  the  rows  of  three  pocket-like 
valves  each  around  the  proximal  end  (semilunar  valves),  and  a 
single  row  of  three  similar  but  larger  valves  at  the  junction  of  the 
conus  and  ventral  aorta.  There  is  some  variability  in  the  number 
of  rows  of  valves  in  the  conus  of  Squalus;  there  are  always  three 
rows  of  three  valves  each  in  that  of  Eugaleus. 

In  the  aorta  notice  the  apertures  without  valves  which  lead  into 
the  afferent  branchial  vessels. 

HEPATIC  PORTAL  SYSTEM.  The  hepatic  portal  vein  is  the  large 
vein  entering  the  liver  alongside  the  hepatic  artery  and  bile  duct.  It 
receives  branches  from  the  stomach,  pancreas,  spleen,  intestine,  and 
rectal  gland. 


16 

At  the  surface  of  the  liver  it  divides  into  two  branches,  which 
enter  the  two  lobes  of  this  organ.  Within  the  liver  the  hepatic 
portal  veins  branch  until  a  capillary  sytem  is  formed  from  which 
the  blood  is  collected  by  the  hepatic  veins  and  carried  into  the  sinus 
venosus. 

In  general,  the  branches  of  origin  of  the  hepatic  portal  vein  fol- 
low closely  the  arteries  of  the  digestive  organs.  Trace  the  follow- 
ing parts  of  the  system :  A  posterior  intestinal  vein,  from  the  rectal 
gland  and  rectum,  the  large  intestine  and  spiral  valve,  across  to  the 
end  of  the  pancreas,  along  the  pancreas  to  the  hepatic  portal  vein; 
an  anterior  intestinal  vein,  from  the  large  intestine  and  spiral  valve, 
along  the  duodenal  lobe  of  the  pancreas;  gastric,  duodenal,  and 
pyloric  veins  joining  the  veins  already  traced;  a  splenic  vein  join- 
ing the  posterior  intestinal  vein. 

The  liver,  with  the  bile  duct,  may  now  be  removed  from  the 
body  if  it  is  desired  to  trace  the  bile  duct  into  the  bladder  or  to 
trace  the  hepatic  ducts.  This  can  be  done  best  by  gently  scraping 
away  the  soft  liver  tissue  until  the  bladder  and  ducts  are  exposed. 

THE  NERVOUS  SYSTEM. 

Only  the  head  and  anterior  part  of  the  trunk  will  be  required  for 
the  dissection  of  the  nervous  system.  Cut  across  the  body  back  of 
the  pectoral  fins;  the  posterior  part  of  the  body  will  not  be  re- 
quired further  unless  it  is  desired  to  study  the  muscles  and  skeleton. 

The  manner  of  dissecting  the  brain  depends  somewhat  upon 
the  specimens  at  the  disposal  of  the  student.  If  a  large  head  is  to 
be  used  especially  for  the  dissection  of  the  cranial  nerves,  only  the 
brain,  eye  and  ear  need  be  studied  in  the  present  specimen.  But  in 
most  cases  it  will  be  found  best  for  the  student  to  dissect  the  first 
dogfish  as  thoroughly  as  possible,  working  out  the  cranial  nerves 
as  well  as  the  brain,  and  reserving  the  second  head  for  a  thorough 
review  of  the  entire  nervous  system.  Chapter  III  of  Herrick  and 
Crosby's  "Laboratory  Outline  of  Neurology"  should  be  used  in  con- 
nection with  such  a  review. 

If  a  line  be  drawn  over  the  dorsal  surface  of  the  head  connect- 
ing the  two  spiracles,  two  small  pores  will  be  found  near  the  middle. 
These  are  the  external  apertures  of  the  ducti  lymphatici.  Cut 
carefully  through  the  skin  in  a  small  circle  around  the  pores,  and 
remove  the  skin  from  the  remainder  of  the  dorsal  surface  of  the 
skull  without  disturbing  the  small  section  containing  the  pores. 
The  latter  part  should  now  be  lifted  gently;  beneath  it  will  be 
seen  two  delicate  tubes  passing  from  the  pores  to  apertures  in  a  de- 
pression of  the  skull  below  them.  These  tubes  are  the  ducti  endo- 
lymphatici,  through  which  a  passage  exists  between  the  internal  ear 
and  the  exterior.  As  they  cannot  be  preserved  in  the  subsequent 
dissection,  the  pores  by  which  they  pass  through  the  skull  to  the  in- 
ternal ear  should  be  found  now,  and  a  memorandum-sketch  made 
of  the  ducts  themselves. 

DORSAL  SURFACE  OF  THE  BRAIN.  The  roof  of  the  skull  should  be 
removed  from  over  the  brain.  Use  a  sharp  scalpel  and  take  very 
thin  slices  of  cartilage.  Do  not  cut  beyond  the  brain  at  the  sides. 
No  attempt  should  be  made  at  this  time  to  expose  more  than  the 
dorsal  surface  of  the  brain. 

Above  the  anterior  end  of  the  brain  there  is  a  small  median 
foramen  through  the  skull,  the  epiphysial  foramen.  A  strand  of 
tissue,  the  epiphysis,  leading  from  this  to  the  surface  of  the  brain, 
should  be  carefully  observed  and  retained.  The  cartilage  should 


17 

also  be  cut  away  from  above  the  portion  of  the  spinal  cord  next 
the  skull.  Gently  wash  away  any  coagulated  lymph. 

The  brain  and  spinal  cord  are  invested  by  two  membranes 
(meninges) .  The  tough  dura  mater  lines  the  cavity  in  which  they 
lie,  clinging  closely  to  the  cartilage;  in  fact  it  forms  the  perichon- 
drium  of  the  internal  surface  of  the  cranium.  The  pia  mater  en- 
velops closely  the  brain  and  cord,  and  contains  numerous  blood 
vessels.  Between  the  two  is  the  arachnoid  space,  traversed  by  oc- 
casional fine  threads  of  connective  tissue  and  filled  with  lymph. 

As  the  spinal  cord  passes  forward  into  the  skull  it  enlarges  and 
merges  with  the  posterior  portion  of  the  brain,  the  medulla  oblon- 
gata  (myelencephalon) .  The  roof  of  the  medulla  is  extremely 
thin,  and  is  broken  if  the  cartilage  has  not  been  removed  with  ex- 
treme care,  exposing  a  cavity  within,  the  fourth  ventricle. 

In  front  of  the  medulla,  and  overlapping  its  anterior  extremity, 
is  a  large  oval  organ,  the  cerebellum  (metencephalon) .  Ventral 
to  the  cerebellum,  each  side  of  the  medulla  is  expanded  in  an  ear- 
shaped  lobe,  the  corpus  restiformis.  Anteriorly,  the  cerebellum 
overlaps  a  pair  of  rounded  lobes,  the  optic  lobes,  which  together 
form  the  dorsal  portion  of  the  midbrain  (mesencephalon) . 

In  front  of  the  optic  lobes  are  two  slightly  larger  lobes  united 
in  their  posterior  portions  but  separated  anteriorly,  the  cerebral 
lobes  or  hemispheres.  Together  they  constitute  the  prosencephalon. 
(The  prosencephalon  is  not  divided  in  Eugaleus.)  Between  the 
mesencephalon  and  the  prosencephalon  is  a  depressed  region  be- 
longing to  the  brain-stem,  the  diencephalon  (thalamencephalon) , 
from  which  the  epiphysis  arises.  The  roof  of  the  diencephalon 
also  is  very  thin  and  is  frequently  broken  during  the  exposure  of 
the  brain.  The  cavity  seen  within  the  diencephalon  is  the  third 
ventricle. 

Stalked  bodies  arising  from  the  antero-lateral  angles  of  the 
cerebral  hemispheres  are  the  olfactory  lobes.  The  portion  of  the 
brain  including  the  cerebral  hemispheres  and  the  olfactory  lobes 
constitutes  the  telencephalon. 

DISSECTION  OF  THE  INTERNAL  EAR.  The  structures  composing 
this  organ  lie  in  the  projecting  cartilage  at  the  side  of  the  medulla 
(auditory  capsule).  Remove  the  cartilage  of  the  auditory  capsule 
in  thin  slices  and  bit  by  bit,  following  the  ductus  endolymphaticus 
to  the  membraneous  labyrinth.  Dissect  away  the  surrounding  car- 
tilage leaving  the  membraneous  canals  in  place,  until  the  entire 
labyrinth  is  exposed.  The  membraneous  labyrinth  consists  of  a 
large  central  sac  (utriculo-saccular  chamber)  into  which  the  endo- 
lymphatic  duct  opens,  and  three  membraneous  tubes  (semicircular 
canals)  external  to  the  chamber  but  communicating  with  it  in  vari- 
ous ways.  Two,  one  anterior  and  the  other  posterior  to  the  sacculus, 
lie  in  a  nearly  vertical  plane  (anterior  and  posterior  semicircular 
canals)  ;  one  is  external  to  the  sacculus  and  lies  in  a  nearly  hori- 
zontal plane  (horizontal  or  external  semicircular  canal).  At  the 
ventral  ends  of  the  vertical  canals  are  nearly  spherical  enlarge- 
ments called  ampullae.  The  ampulla  of  the  horizontal  canal  is  at 
its  anterior  end.  The  dorsal  ends  of  the  vertical  canals  open  near 
each  other  into  the  upper  part  of  the  utriculo-sacculus.  The  ven- 
tral extremity  of  the  anterior  vertical  canal  and  the  anterior  ex- 
tremity of  the  horizontal  canal  open  beside  each  other  into  an  an- 
terior projection  of  the  sacculus.  The  ventral  extremity  of  the  pos- 
terior vertical  canal  opens  into  the  posterior  and  lower  part  of  the 
sacculus.  The  posterior  extremity  of  the  horizonal  canal  opens 


18 

into  the  posterior  side  of  the  sacculus.  During  life  the  utrici 
sacculus  and  the  semicircular  canals  are  filled  with  a  lymph 
fluid,  and  the  sacculus  contains  a  large  calcerous  ear-stone  (otolii 
which  is  usually  dissolved  by  the  formalin  used  in  preserving 
dogfish. 

Whitish  patches  of  thickened  sensory  epithelium  may  be  seei 
the  ampullae  (cristate  acusticae)  and  in  the  utriculo-saccular  ch 
her  (maculae  acusticae).    Branches  of  the  eighth  nerve  can  be 
lowed  to  all  these  areas. 

A  projection  of  the  ventral  wall  of  the  utriculo-sacculus  is 
lagena,  the  rudiment  from  which  the  cochlea  of  higher  anin 
developed.  It  also  contains  a  macula  acustica. 

EXTERNAL  FEATURES  OF  THE  EYE.  Observe  the  transpa: 
cornea  covering  the  external  surface  of  the  eye;  the  dark  ring 
the  iris;  the  central  opening  in  the  iris,  the  pupil;  the  conjur 
val  sac  surrounding  the  external  half  of  the  eyeball.  Cut  a 
sufficient  of  the  upper  wall  of  the  cartilaginous  orbit  to  expose 
eyeball  and  its  muscles.  Note  the  considerable  amount  of  soft  < 
nective  tissue  around  the  eye  and  explore  the  orbital  sinus  (p.  ] 
Take  notice  of  the  following  nerves,  in  order  to  ensure  their  pre 
vation  until  the  time  comes  to  trace  them  more  completely.  A  h 
nerve  crossing  the  medial  side  of  the  orbit,  the  superficial  opht 
mic;  a  nerve  leaving  the  cranium  opposite  the  optic  lobe,  pas: 
under  the  superficial  ophthalmic  to  the  anterior  muscle  of  the 
ball,  the  trochlear;  several  long  ciliary  nerves  passing  to  the 
ball;  several  other  nerves  visible  in  the  deep  angle  of  the  orbit. 

Six  muscles  move  the  eye.  Four  of  these  arise  close  toge 
at  the  deep  postero-medial  angle  of  the  orbit.  Diverging,  they 
inserted  upon  four  sides  of  the  eyeball,  and  from  the  positior 
their  insertions  are  named  the  superior,  posterior,  inferior, 
anterior  recti.  Two  muscles  arise  from  the  antero-medial  angl< 
the  orbit,  the  superior  and  inferior  oblique  muscles. 

Between  the  recti  muscles  can  be  seen  a  mushroom-shaped  s 
of  cartilage,  the  ophthalmic  peduncle;  the  eyeball  rests  againsi 
expanded  end.  (There  is  no  peduncle  in  Eugaleus.) 

THE  CRANIAL  NERVES.  The  cranial  nerves  are  twelve  pair; 
nerves  arising  from  the  brain,  and  thus  distinguished  from  the  sp 
nerves  which  arise  from  the  sides  of  the  spinal  cord.  They  are 
tributed  chiefly  to  the  head  and  neck,  though  branches  of  the  vi 
nerve  go  to  the  viscera  and  to  the  sense  organs  of  the  lateral  ] 
Since  the  nerves  are  all  paired,  the  distribution  of  both  nerves  < 
pair  being  alike,  the  descriptions  will  mention  but  one  nerve  < 
pair.  As  the  cranial  nerves  are  traced  dissect  away  the  sides  of 
cranium  down  to  the  foramina  penetrated  by  the  nerves,  and  fol 
each  nerve  from  its  origin  on  the  brain  to  the  parts  innervated  b 
Features  of  the  dissection  which  are  not  found  in  tracing  the  n< 
of  one  side  should  be  sought  on  the  other  side. 

The  olfactory  nerve.    The  anterior  surface  of  the  olfactory  1 
fills  a  large  foramen  in  the  anterior  wall  of  the  cranium  am 
pressed  closely  against  the  posterior  surface  of  the  nasal  sac. 
merous  small  nerves,  collectively  forming  the  olfactory  nerve,  a 
from  the  anterior  face  of  the  lobe,  penetrate  the  membraneous  i 
of  the  olfactory  organ,  and  are  distributed  to  its  highly  folded 
face. 

The  terminal  nerve,  Nervus  terminalis,  is  a  slender  nerve  : 
ning  along  the  medial  surface  of  the  stalk  of  the  olfactory  1< 
Follow  it  backward  to  its  origin  on  the  anterior  surface  of  the  c 


19 

bral  hemisphere,  deep  in  the  median  fissure  (in  Eugaleus  on  the 
ventral  surface).  Trace  it  forward  aver  the  dorsal  surface  of  the 
olfactory  lobe  to  where  it  enters  the  nasal  sac.  The  terminal  nerve 
is  a  true  cranial  nerve  which  has  escaped  notice  until  recent  years. 
It  is  associated  with  the  olfactory  nerve  in  vertebrates  generally  from 
fishes  to  men.  The  fibres  of  the  terminal  nerve  remain  distinct  from 
those  of  the  olfactory  nerve,  both  in  the  olfactory  organ  and  in  the 
brain.  Its  function  is  unknown.  There  is  still  a  division  of  opinion 
among  authorities  as  to  whether  the  terminal  nerve  should  be  con- 
sidered to  be  a  distinct  cranial  nerve,  or  a  portion  of  the  olfactory 
nerve. 

The  optic  nerve  can  be  seen  at  the  bottom  of  the  orbit  between 
the  eye  and  the  skull,  nearly  under  the  superior  oblique  muscle.  It 
arises  from  the  ventral  side  of  the  diencephalon,  passes  outward, 
penetrates  the  orbit  at  its  infero-medial  angle,  and  continues  directly 
outward  to  the  eyeball. 

The  trochlear  nerve,  or  patheticus,  penetrates  the  wall  of  the 
orbit  opposite  the  optic  lobe.  Follow  it  back  to  its  origin  from  the 
dorsal  surface  of  the  brain  in  the  depression  between  the  optic 
lobes  and  the  cerebellum.  Then  follow  it  from  the  skull  to  the 
superior  oblique  muscle,  which  it  innervates. 

The  oculo-motor  nerve  arises  from  the  ventral  surface  of  the 
midbrain,  passes  outward,  and  penetrates  the  orbit  on  a  level  with 
and  just  anterior  to  the  origins  of  the  recti  muscles.  It  divides  im- 
mediately into  three  parts;  two  pass  to  the  anterior  and  superior 
recti  respectively,  while  the  third  passes  downward  along  the  pos- 
terior surface  of  the  eyeball  to  the  inferior  rectus  and  inferior 
oblique  muscles.  In  tracing  this  nerve  the  palatine  process  of  the 
upper  jaw  will  be  seen  projecting  from  below  into  the  orbit. 

The  trigeminal,  facial,  and  auditory  nerves  spring  from  the  side 
of  the  medulla  below  the  corpus  restiformis.  The  roots,  and  some 
of  the  branches,  of  the  trigeminal  and  facial  nerves  are  so  mingled 
as  to  be  indistinguishable  except  by  special  neurological  technique. 
The  common  root  of  the  trigeminal  and  facial  nerves  shows  a  par- 
tial division  into  a  dorsal  and  a  ventral  portion;  the  dorsal  portion 
belongs  to  the  facial  nerve,  while  the  ventral  root  is  mixed.  The 
root  of  the  auditory  nerve  lies  close  behind  the  trigeminal-facial 
root,  but  can  be  distinguished  fairly  well.  Both  the  trigeminal  and 
facial  nerves  divide  into  several  trunks,  namely: 

Trigeminal  Facial 

superficial  ophthalmic  superficial  ophthalmic 

deep  ophthalmic  buccal 

maxillary  otic 

mandibular  hyomandibular 

The  superficial  ophthalmic  trunks  of  the  two  unite  in  a  single 
nerve  which  passes  along  the  inner  wall  of  the  orbit  above  the 
muscles  of  the  eye  to  a  foramen  in  the  antero-medial  angle  of  the 
orbit,  through  which  it  passes  to  the  dorsal  surface  of  the  snout. 
The  superficial  ophthalmic  nerve  of  Squalus  is  composed  almost 
entirely  of  fibres  of  the  facial  nerve.  The  superficial  ophthalmic 
trunk  of  the  trigeminal  gives  rise  to  several  small  nerves  leaving 
the  common  trunk  near  its  origin  and  passing  to  the  skin  above  the 
eye.  The  superficial  ophthalmic  trunk  of  the  facial,  nearly  the 
whole  of  the  common  nerve,  branches  profusely  to  supply  the 
sensory  organs  of  the  dorsal  and  lateral  surfaces  of  the  snout. 

The  superficial  ophthalmic  of  Eugaleus  rises  from  the  dorsal  part  of  the 
trigemino-facial  root  and  leaves  the  cranium  by  a  separate  foramen  above  and 
anterior  to  the  roots  of  the  recti  muscles. 


20 

Directly  under  the  origin  of  the  superficial  ophthalmic  will  be 
found  a  comparatively  slender  nerve,  which  passes  between  the 
superior  and  posterior  rectus  muscles,  and  forward  along  the  medial 
surface  of  the  eyeball;  it  penetrates  the  anterior  wall  of  the  orbit 
by  a  separate  foramen,  and  emerges  under  the  superficial  ophthal- 
mic. It  is  distributed  to  the  skin  of  the  dorsal  and  lateral  surfaces 
of  the  snout.  This  is  the  deep  ophthalmic  (ophthalmicus  profundus) 
of  the  trigeminal  nerve.  A  slender  branch  (posterior  ciliary  nerve) 
passes  from  the  deep  ophthalmic  near  its  origin  to  the  posterior  sur- 
face of  the  eyeball.  Farther  forward  the  same  trunk  gives  off  an 
anterior  ciliary  nerve  to  the  anterior  part  of  the  eyeball. 

A  large  nerve  which  crosses  the  floor  of  the  orbit,  beneath  the 
eyeball,  consists  of  the  maxillary  trunk  of  the  trigeminal  and  the 
buccal  trunk  of  the  facial  nerve.  These  remain  associated,  even 
into  the  small  branches.  Near  the  anterior  margin  of  the  orbit  the 
maxillary-buccal  trunk  divides  into  three  parts;  the  smallest  and 
outer  one  passes  to  the  surface  lateral  and  anterior  to  the  eye.  The 
other  two  dip  downward  and  pass  in  front  of  the  jaw  to  the  ventral 
surface  of  the  snout.  Reflect  the  skin  of  the  ventral  surface  of  the 
snout,  and  by  dissection  expose  these  nerves  as  they  emerge  from 
the  orbit.  The  larger  branch  runs  forward  close  to  the  median  line 
of  the  snout,  giving  off  numerous  twigs;  the  other,  which  appears 
to  be  a  pure  trigeminal  branch,  is  distributed  near  the  angle  of  the 
mouth.  The  fibres  of  the  maxillary  trunk  supply  the  skin,  while 
those  of  the  buccal  go  to  the  canal  organs  and  ampullae  of  Loren- 
zini. 

The  mandibular  trunk  of  the  trigeminal  nerve  arises  beneath 
and  behind  the  maxillary.  It  passes  outward  in  front  of  the  levator 
maxillae  superioris  muscle,  sending  a  few  twigs  into  this  muscle, 
and  turns  downward  over  the  palato-quadrate  cartilage.  It  di- 
vides here,  one  part  entering  the  adductor  mandibularis  muscle,  the 
other  passing  downward  along  the  edge  of  the  mandible,  innervating 
the  skin  of  the  lower  jaw  and  the  first  ventral  superficial  constrictor 
muscle. 

The  mandibular  and  maxillary-buccal  trunks  of  Eugaleus  are  united  until 
they  approach  the  edge  of  the  orbit,  and  the  palatine  branch  is  much  larger; 
otherwise  the  trigemino-facial  branches  are  much  as  in  Squalus. 

The  hyomandibular  trunk  of  the  facial  nerve  can  be  found  just 
beneath  the  skin  behind  and  close  to  the  spiracle.  From  here  it 
can  be  followed  back  to  the  brain.  It  arises  from  the  ventral  part 
of  the  trigemino-facial  root,  emerging  from  the  cranium  through 
the  hyomandibular  canal.  It  divides  into  a  number  of  branches 
just  beyond  the  spiracle: 

1.  The  external  mandibular  branch  consists  of  two  portions,  a 
small  anterior  nerve  extending  antero-ventrally  to  the  canals  above 
and  below  the  angle  of  the  mouth,  and  a  larger  nerve  which  passes 
laterally  and  suddenly  breaks  up  into  a  brush  of  twigs  which  in- 
nervate the  hyoidean  group  of  ampullae. 

2.  The  internal  mandibular  branch  arises  at   about  the  same 
level  as  the  external  mandibular,  but  under  it,  passes  inward  around 
the  edge  of  the  hyoid  cartilage,  under  the  adductor  mandibularis 
muscle,  and  then  forward  along  the  mandibular  cartilage. 

3.  The  hyoid  branch  separates  from  the  hyomandibular  trunk 
at  about  the  same  level  as  the  preceding  nerves,  and  then  passes, 
deep  in  the  tissues,  around  the  angle  of  the  jaw  to  the  ventral  side 
where  it  is  distributed  to  the  superficial  constrictor  muscles.     Sev- 
eral nerves  pass  from  the  hyomandibular  trunk  and  the  hyoid  branch 
to  the  dorsal  superficial  constrictors. 


21 

4.  The  palatine  branch  springs  from  the  base  of  the  hyoman- 
dibular  trunk  inside  the  hyomandibular  canal.  It  passes  outward 
and  forward,  dividing  into  numerous  branches  which  innervate  the 
mucous  membranes  of  the  mouth.  It  can  be  traced  completely  later. 

One  or  more  small  nerves  proceeding  to  the  pseudobranch  and 
anterior  wall  of  the  spiracle  arise  near  the  point  of  origin  of  the 
palatine  branch. 

The  otic  nerve,  passing  from  the  root  of  the  facial  nerve  to  the 
postorbital  canal,  is  not  likely  to  be  found  in  this  dissection. 

Observe  the  enlargement  near  the  base  of  the  hyomandibular 
trunk,  and  within  the  cartilaginous  canal,  the  geniculate  ganglion. 
The  gasserian  ganglion,  a  component  of  the  trigeminal  nerve,  lies  in 
the  ventral  portion  of  the  trigeminal-facial  root,  and  can  now  be 
located. 

The  auditory  nerve  arises  close  behind  the  ventral  division  of 
the  trigemino-facial  root.  The  root  of  the  auditory  nerve  encloses 
a  large  auditory  ganglion.  A  vestibular  nerve  arises  from  the  an- 
terior end  of  the  auditory  ganglion  and  passes  into  the  ear  capsule, 
innervating  the  upper  part  of  the  utriculo-sacculus  and  the  ampullae 
of  the  anterior  and  horizontal  canals.  From  the  posterior  part  of 
the  ganglion  nerves  pass  to  the  ventral  part  of  the  sacculus  and  the 
ampulla  of  the  posterior  canal.  Trace  these  branches  as  thor- 
oughly as  possible. 

The  abducens  nerve  emerges  from  the  cranium  under  and  close 
to  the  origin  of  the  posterior  rectus  muscle,  into  which  muscle  it 
enters.  To  expose  this  nerve  the  trigeminal,  facial  and  auditory 
nerves  must  be  lifted  and  cut  as  they  pass  through  the  wall  of  the 
cranium.  It  can  be  traced  obliquely  backward  and  inward,  through 
a  long  canal,  to  its  origin  near  the  mid-line  of  the  ventral  surface 
of  the  medulla. 

The  glossopharyngeal  nerve  passes  through  the  base  of  the  ear 
capsule  from  the  side  of  the  medulla  to  the  upper  end  of  the  first 
branchial  pouch.  A  ganglionic  enlargement  is  found  near  where  it 
emerges  from  the  cartilage.  Outside  the  cranium  the  glossopharyn- 
geal divides  into  a  pretrematic  branch,  passing  down  in  front  of 
the  first  gill  pouch,  and  a  posttrematic  branch  running  behind  the 
pouch.  The  pretrematic  branch  quickly  sends  off  a  pharyngeal 
nerve  which  runs  antero-ventrally  to  the  roof  of  the  pharynx.  The 
pretrematic  and  posttrematic  branches  can  be  followed  along  the 
gill-arch  to  the  ventral  side  of  the  pharynx.  A  fourth  branch  of 
the  glossopharyngeal,  the  supratemporal,  springs  from  the  dorsal 
side  of  the  ganglion;  passing  through  the  ear  capsule  it  runs  to  the 
dorsal  surface  of  the  head,  where  it  is  distributed  to  the  sense  organs 
of  a  short  section  of  the  lateral  line  canal.  This  small  nerve  can 
be  demonstrated  by  carefully  separating  the  muscles  and  perichon- 
drium  from  the  posterior  surface  of  the  auditory  capsule. 

The  vagus  nerve  (or  pneumo gastric)  arises  by  an  extensive 
series  of  roots  from  the  side  of  the  medulla.  An  easily  distinguished 
ribbon-like  portion  of  the  root,  the  lateral  line  root,  runs  forward 
as  far  as  the  root  of  the  glossopharyngeal.  Note  the  canal  by  which 
the  vagus  leaves  the  cranium,  and  trace  the  nerve  along  the  inner 
side  of  the  anterior  cardinal  vein. 

The  principal  branches  of  the  vagus  are: 

1.  The  supratemporal  branch,  a  small  nerve  running  dorsal 
through  the  posterior  part  of  the  ear  capsule  to  the  lateral  line 
canal  and  other  sense  organs  of  the  head.  It  will  be  found  near  the 
supratemporal  branch  of  the  glossopharyngeal. 


22 

2.  The  lateral  line  branch,  a  large  nerve  which  separates  from 
the  trunk  of  the  vagus  just  outside  the  cranium  and  runs  backward 
through  the  muscles,  parallel  to  the  vertebral  column  on  a  level 
with  the  lateral  line.     It  sends  off  numerous  small  twigs  to  the 
sense  organs  of  the  lateral  line  canal. 

3.  Four  branchial  nerves,  which  can  be  seen  through  the  floor 
of  the  anterior  cardinal  vein,  leave  the  outer  side  of  the  vagus 
trunk.    Each  divides  into  a  pretrematic  and  posttrematic  branch;  a. 
pharyngeal  branch,  the  last  of  which  is  the  largest,  arises  from 
each  posttrematic. 

4.  Beyond   the   branchial   nerves   the   remainder   of   the  vagus 
passes  backward  as  the  intestinal  or  visceral  trunk,  to  the  end  of 
the  pharynx,  where  it  divides  into  a  number  of  branches  which  are 
distributed  chiefly  to  the  wall  of  the  stomach.     Near  the  point  of 
this  last  division  the  vagus  is  crossed  by  the  hypobranchial  nerve, 
which  should  be  noted  and  preserved. 

The  occipital  nerve  penetrates  the  lateral  wall  of  the  cranium 
close  behind  the  root  of  the  vagus  and  enters  the  canal  of  the  vagus, 
along  which  it  passes.  On  emerging,  it  sends  small  branches  to  the 
nearby  muscle,  while  the  principal  portion  runs  on  to  join  the 
hypobranchial  nerve.  The  occipital  nerve  will  be  found  to  arise 
from  the  ventral  surface  of  the  medulla,  below  and  behind  the  root 
of  the  vagus,  by  two  or  more  distinctly  separated  roots,  which  may 
represent  distinct  nerves. 

SPINAL  NERVES.  The  spinal  nerves  are  those  nerves  which  arise 
from  the  sides  of  the  spinal  cord.  They  differ  from  the  cranial 
nerves  not  only  in  their  origin  outside  the  cranium,  but  also  in 
that  each  spinal  nerve  arises  by  two  roots  which  spring  from  the 
spinal  cord  near  the  dorsal  and  ventral  surfaces.  Each  root  passes 
through  a  foramen  in  the  cartilaginous  wall  of  the  neural  canal, 
the  ventral  a  little  anterior  to  the  dorsal,  after  which  they  unite  to 
form  the  spinal  nerve.  Between  the  junction  of  the  roots  and  its 
foramen  the  dorsal  root  contains  a  mass  of  ganglion  cells,  which 
cause  an  enlargement  known  as  the  dorsal  root  ganglion.  The 
typical  course  of  a  spinal  nerve  is  around  the  body  to  the  ventral 
surface,  giving  off  branches  to  the  muscles  and  skin  of  its  segment. 
A  short  distance  from  the  vertebral  column  the  spinal  nerves  lie  just 
outside  the  peritoneum,  through  which  many  of  them  can  be  seen 
and  followed  to  about  the  level  of  the  lateral  vein.  At  this  point 
they  pass  outward  into  the  muscles  of  the  body  wall.  To  dissect 
any  of  the  spinal  nerves  make  a  longitudinal  incision  along  the 
lateral  line  and  separate  the  dorsal  muscle  mass  from  the  lateral 
muscles  for  some  distance.  The  dorsal  muscles  can  then  be  pressed 
toward  the  vertebral  column  and  dissected  away  from  the  perito- 
neum. The  spinal  nerves,  lying  against  the  peritoneum,  will  be 
exposed  and  can  be  followed  easily,  first  to  their  roots,  next  ven- 
trally. 

The  hypobranchial  nerve,  to  which  attention  was  called  at  the 
point  where  it  crosses  the  vagus,  is  formed  by  the  union  of  the 
principal  branches  of  the  occipital  and  first  two  spinal  nerves.  The 
third  spinal  nerve  receives  a  branch  from  the  second,  and  itself 
accompanies  the  hypobranchial  nerve  closely  without  actually  be- 
coming a  part  of  it.  The  union  of  nerves  thus  formed  is  known  as 
a  plexus.  After  crossing  the  vagus  the  hypobranchial  nerve  forks, 
one  division  passing  medial  to,  the  other  lateral  to  the  anterior 
cardinal  vein;  both  run  ventrally,  following  the  last  gill  arch,  and 
reunite  on  the  lateral  wall  of  the  pericardium,  forming  a  trunk 


23 

which  runs  forward.  At  the  anterior  end  of  the  pericardium  this 
divides  into  a  dorsal  and  a  ventral  branch  which  innervate  the  sur- 
rounding muscles.  The  hypobranchial  nerve  innervates  the  skin 
of  the  region  immediately  in  front  of  the  pectoral  girdle,  and  the 
coraco-arcualis  communis,  coracomandibularis,  coracohyoideus,  and 
coracobranchialis  muscles. 

The  third,  fourth,  fifth  and  sixth  spinal  nerves  pass  backward 
and  ventrad  till  they  reach  the  level  of  the  articulation  of  the  pec- 
toral fin  with  the  girdle.  Here  they  join  to  form  a  simple  brachial 
plexus,  from  which  arise  branches  proceeding  to  the  musculature 
of  the  dorsal  and  ventral  faces  of  the  fin.  The  seventh  to  eleventh 
spinal  nerves  pass  downward  to  the  level  of  the  fin,  and  then  branch, 
one  portion  entering  the  muscles  of  the  ventral  body  wall,  while  the 
other  passes  into  the  depressor  muscles  of  the  fin. 

The  pelvic  fin  is  innervated  by  eight  or  nine  spinal  nerves  which 
pass  backward  and  downward  along  the  medial  edge  of  the  septum 
between  the  myomeres,  entering  the  dorsal  side  of  the  fin  along  its 
axis.  No  plexus  is  formed. 

OLFACTORY  ORGAN  (NASAL  SAC).  Dissect  away  the  skin  and  other 
tissues  around  the  nostril  so  as  to  expose  completely  the  olfactory 
organ;  this  will  be  found  to  be  a  dark-colored,  nearly  spherical 
mass,  of  half  the  diameter  of  the  eye,  firmly  attached  at  its  base. 
By  cutting  away  the  cartilage  dorsal  to  the  nasal  sac  its  base  will  be 
exposed,  and  the  olfactory  bulb  will  be  shown  to  be  closely  adherent 
to  a  considerable  part  of  the  postero-dorsal  surface  of  the  organ. 
Numerous  short  nerves  can  be  demonstrated  to  pass  from  the  olfac- 
tory bulb  into  the  olfactory  organ;  all  these  nerves  together  are 
considered  as  the  first  cranial  or  olfactory  nerve.  Remove  the 
olfactory  organ  from  the  head;  divide  it  by  a  median  longitudinal 
cut;  observe  the  arrangement  and  structure  of  its  double  series  of 
internal  folds  (lamellae),  and  the  complete  median  septum. 

VENTRAL  SURFACE  OF  THE  BRAIN.  Cut  the  cord  in  two  some  dis- 
tance back  of  the  brain.  Cut  all  cranial  nerves  just  inside  the 
cranium  and  carefully  lift  the  brain  out.  Parts  of  the  ventral  por- 
tion of  the  brain  lie  in  a  recess  beneath  the  mesencephalon  and 
must  be  disengaged  very  gently. 

Identify  and  examine  the  ventral  parts  of  the  brain.  Note  the 
considerable  lateral  compression  of  the  mesencephalon.  The  optic 
nerves  cross  beneath  the  diencephalon,  forming  the  optic  chiasma. 
From  the  sides  of  the  chiasma  slightly  elevated  optic  tracts,  formed 
by  the  fibres  of  the  optic  nerves,  can  be  traced  into  the  optic  lobes. 

Back  of  the  optic  chiasma  the  projecting  ventral  portion  of  the 
diencephalon  forms  the  hypothalamus.  The  posterior  lobe  of  this 
structure  is  the  hypophysis  or  pituitary  body. 

The  oculomotor  nerves  emerge  over  the  posterior  end  of  the 
hypothalamus. 

The  ventral  portion  of  the  mesencephalon  is  formed  by  the 
cerebral  peduncles  (crura  cerebri) ,  columns  of  fibres  passing  be- 
tween the  myelencephalon  and  telencephalon. 

The  abducens  nerves  arise  on  the  ventral  surface  of  the  myelen- 
cephalon near  the  midline  and  just  back  of  a  line  connecting  the 
roots  of  the  auditory  nerves. 

The  internal  carotid  arteries  reach  the  brain  at  the  sides  of  the 
hypothalamus.  Branches  are  sent  upward  and  forward  over  the 
surface  of  the  brain.  Anastomoses  between  the  vessels  of  the  op- 
posite sides  are  formed  anterior  to  the  optic  chiasma.  The  main 
branches  of  the  carotids  pass  backward  along  the  sides  of  the  hypo- 


24 

thalamus  and  unite  behind  this  organ.  The  median  artery  thus 
formed  runs  along  the  ventral  surface  of  the  myelencephalon  and 
the  spinal  cord.  Numerous  transverse  vessels  are  given  off  to  the 
myelencephalon. 

Identify  the  roots  of  the  remaining  cranial  nerves. 

CAVITIES  OF  THE  BRAIN.  Divide  the  brain  into  exactly  equal 
halves  by  a  vertical  longitudinal  cut. 

Each  lobe  of  the  prosencephalon  contains  a  large  cavity.  These 
are  the  prosocoels.  They  are  commonly  known  either  as  the  lateral 
ventricles,  or  the  left  cavity  as  the  first  ventricle  and  the  right  as 
the  second  ventricle.  The  prosocoels  are  continued  into  the  olfac- 
tory lobes,  these  portions  being  known  as  rhinocoels. 

The  thalamocoel  is  the  cavity  within  the  diencephalon,  often 
called  the  third  ventricle.  The  prosocoels  communicate  with  the 
thalamocoel  by  lateral  openings,  the  foramina  of  Monro.  The  roof 
of  the  thalamocoel  is  very  thin  and  is  non-nervous;  it  is  frequently 
torn  during  the  early  dissection.  Where  the  lobes  of  the  prosence- 
phalon meet  the  dorsal  wall  of  the  diencephalon  this  thin  roof  is 
pushed  into  the  prosocoels,  carrying  with  it  the  pia  mater  and  its 
blood  vessels,  and  thus  forms  vascular  ingrowths  known  as  the 
choroid  plexi.  The  thalamocoel  continues  above  into  the  epiphysis 
and  below  into  the  hypothalamus. 

The  myelocoel  is  the  large  cavity  of  the  myelencephalon.  It 
also  is  frequently  apparently  open  to  the  exterior  at  the  posterior 
end  by  the  accidental  breaking  of  the  thin,  non-nervous  dorsal  wall 
of  this  region.  The  myelocoel  is  also  known  as  the  fourth  ventricle. 

The  thalamocoel  and  myelocoel  are  connected  by  a  narrow  pas- 
sage through  the  mesencephalon,  the  aqueduct  of  Sylvius  (iter, 
mesocoel) . 

The  optocoels  are  large  cavities  within  the  optic  lobes  which 
open  into  the  aqueduct  of  Sylvius. 

A  large  metacoel  in  the  metencephalon  opens  into  the  myelo- 
coel. The  myelocoel  is  also  continued  into  the  corpora  restiforma; 
posteriorly  it  joins  the  central  canal  which  extends  down  the  center 
of  the  spinal  cord. 

DISSECTION  OF  THE  EYE.  Remove  one  of  the  eyes  from  its  orbit, 
and  divide  it  into  inner  and  outer  halves  by  an  equatorial  cut  around 
the  eyeball  (not  directly  through  it,  as  this  tears  the  lens  from  its 
fastenings).  Place  the  halves  under  water  and  observe: 

In  the  inner  half: 

The  posterior  chamber,  the  cavity  of  the  eyeball  which  has  been 
opened.  During  life  it  is  filled  by  a  gelatinous  substance,  the 
vitreous  humor. 

The  retina,  a  delicate  yellowish-white  membrane  lining  the 
interior  of  the  eye,  loosely  attached  to  the  outer  coats  except  at  the 
point  of  entrance  of  the  optic  nerve. 

The  choroid  coat,  a  thin,  black  membrane  outside  the  retina.  It 
can  be  pulled  away  from  the  outer  coat  quite  easily  except  near  the 
optic  nerve. 

The  sclerotic  coat,  the  outer  coat  of  the  eye.  This  is  composed 
of  connective  tissue  having  an  almost  cartilaginous  consistency,  is 
only  slightly  pigmented,  and  is  somewhat  translucent.  The  muscles 
of  the  eye  are  inserted  upon  the  sclerotic. 

In  the  outer  half: 

The  ora  serrata,  an  irregular  line  along  which  the  retina  ends. 

The  iris,  a  fold  of  the  choroid  extending  inward  like  a  shelf,  and 


25 

perforated  centrally  to  form  the  pupil.  Around  the  iris  the  choroid 
is  folded  radially  into  the  ciliary  processes. 

The  lens,  a  spherical  body,  transparent  and  elastic  during  life, 
but  opaque  and  hard  in  preserved  specimens.  It  projects  into  the 
pupil  and  is  suspended  from  the  ciliary  processes  by  a  delicate 
membrane,  the  suspensory  ligament. 

The  anterior  chamber,  in  front  of  the  iris  and  lens,  filled  with 
a  watery  fluid,  the  aqueous  humor. 

The  transparent  cornea,  forming  the  outer  side  of  the  eyeball, 
continuous  with  the  sclerotic. 

Take  out  the  other  eye  and  cut  it  in  two  by  a  section  through  the 
pupil  and  optic  nerve.  Review  the  relation  of  the  parts. 

THE  SKELETON 

There  seems  to  be  no  easy  way  of  cleaning  the  skeleton  of  dog- 
fish which  have  been  preserved  in  formalin  or  alcohol,  the  only 
procedure  being  to  cut,  pick,  and  scrape  the  flesh  away  from  the 
skeleton.  Time  and  patience  are  required,  but  if  these  are  allowed 
there  is  no  reason  why  all  the  parts  of  the  skeleton  cannot  be  thor- 
oughly studied.  Specimens  which  have  been  preserved  in  brine  are 
more  easily  skeletonized. 

The  skeleton  is  entirely  composed  of  cartilage  which,  in  large 
species  of  elasmobranchs  and  in  old  individuals  of  small  species, 
becomes  impregnated  with  lime  salts,  in  some  cases  to  such  an  ex- 
tent as  to  resemble  soft  bone. 

The  parts  of  the  skeleton  are  frequently  grouped  under  two 
heads:  the  axial  skeleton,  comprising  the  skull  and  vertebral  col- 
umn; and  the  appendicular  skeleton,  including  the  pectoral  and 
pelvic  girdles  and  the  skeleton  of  the  fins. 

VERTEBRAL  COLUMN.  The  vertebral  column  is  divided  into  two 
regions,  thoracic  and  caudal,  distinguished  by  the  slightly  different 
character  of  the  vertebrae.  Remove  the  muscle  and  connective  tissue 
from  the  vertebral  column  for  a  short  distance  anterior  to  the  first 
dorsal  fin.  Care  is  required  not  to  cut  away  small  cartilages  occu- 
pying the  positions  of  ribs.  Now  remove  from  the  body  about  two 
inches  of  the  portion  of  the  column  exposed  with  any  cartilaginous 
parts  which  may  be  attached  to  the  vertebrae.  The  vertical  column 
is  made  up  of  segments,  called  vertebrae.  Each  vertebra  consists 
of  a  large  ventral  mass,  the  centrum,  and  an  arch,  the  neural  arch, 
roofing  over  the  dorsal  surface  of  the  centrum;  the  arch  is  com- 
posed of  several  small  plates  of  cartilage.  The  opening  enclosed  by 
each  centrum  and  its  neutral  arch  is  the  vertebral  foramen;  the 
joined  vertebral  foramina  form  the  neural  canal,  which  is  occu- 
pied by  the  spinal  cord. 

Separate  one  of  the  vertebrae  from  the  rest.  The  centrum  is 
deeply  concave  at  each  end;  such  a  centrum  is  termed  amphicoelous. 
At  the  middle  of  the  centrum  the  concavities  meet  and  thus  a  canal 
is  formed  through  it.  This  canal  and  the  spaces  between  the  ends 
of  adjoining  vertebrae  are  filled  by  the  remains  of  the  notochord,  a 
rather  pulpy  structure  extending  from  end  to  end  of  the  vertebral 
column. 

The  concave  faces  of  the  vertebrae  consist  of  much  firmer  car- 
tilage than  the  remaining  portions,  sometimes  even  calcified.  Make 
a  transverse  section  through  the  middle  of  a  centrum  and  observe 
the  relations  of  the  parts. 

On  each  side  of  the  centrum,  near  the  ventral  edge,  is  a  plate- 


26 

like  projection,  the  transverse  process.  Attached  to  the  extremity 
of  this  is  a  slender  cartilaginous  rib. 

Each  neural  arch  is  made  up  of  two  distinguishable  sets  of 
plates.  The  first  consists  of  a  pair  of  broad  neural  plates  extend- 
ing upward  from  each  side  of  the  centrum  and  uniting  with  each 
other  dorsally.  Between  the  neural  plates  of  two  successive  verte- 
brae is  a  pair  (one  on  each  side)  of  intercalary  plates  which  also 
unite  over  the  neural  canal.  The  intercalary  plates  are  over  the 
joint  between  the  centra.  Neural  and  intercalary  plates  together 
make  the  lateral  and  dorsal  walls  of  the  neural  canal.  The  rela- 
tions of  these  plates  can  sometimes  be  seen  best  when  the  neural 
arch  is  cleaned,  then  cut  away  from  the  centrum,  and  looked  through 
toward  the  light. 

In  the  lower  part  of  each  neural  plate  is  a  small  foramen  which 
allows  the  passage  of  the  ventral  root  of  the  spinal  nerve.  A  fora- 
men for  .the  dorsal  root  is  found  at  about  the  middle  of  the  inter- 
calary plate. 

Clean  and  remove  some  of  the  caudal  vertebrae  from  the  region 
just  back  of  the  cloaca.  In  general  they  have  nearly  the  same  struc- 
ture and  relations  as  the  thoracic  vertebrae,  but  have  no  transverse 
processes  and  the  plates  of  the  neural  arches  are  not  so  distinct. 
There  is  also  added  a  ventral  arch  similar  in  form  to  the  neural 
arch.  This  is  the  haemal  arch,  in  which  lie  the  caudal  aorta  and 
vein.  Its  roof  is  the  surface  of  the  centrum,  the  sides  are  formed 
by  pairs  of  plates  which  correspond  in  number  to  the  centra,  and 
unite  with  each  other  ventrally.  Between  the  successive  plates  are 
openings  for  the  passage  of  branches  of  the  artery  and  vein. 

In  this  region  foramina  for  the  roots  of  the  spinal  nerves  are 
found  only  in  every  other  pair  of  neural  and  intercalary  plates. 
Toward  the  tip  of  the  vertebral  column  the  relation  of  the  neural 
and  intercalary  plates  to  the  centra  becomes  very  irregular. 

In  Eugaleus  the  roof  of  the  neural  arch  is  formed  by  a  row  of  small,  dia- 
mond-shaped plates  which  fit  in  between  the  other  two  sets.  As  these  plates 
correspond  morphologically  to  the  neural  spines  of  higher  vertebrates,  they  may 
receive  that  name  here.  It  is  probable  that  the  dorsal  portion  of  the  arch  in 
Squalus  is  composed  of  similar  neural  spine  elements  which  have  become  fused 
with  the  neural  and  intercalary  plates  of  each  side. 

SKULL.  The  skull  is  entirely  cartilaginous,  and  comprises  three 
principal  divisions:  (1)  the  cranium,  an  undivided  mass  of  car- 
tilage lodging  the  brain  and  the  organs  of  smell,  sight,  and  hear- 
ing; (2)  the  jaws;  (3)  the  visceral  arches,  or  skeletons  of  the  gill- 
arches. 

(1)  The  cranium.  A  blunt  prolongation  of  the  anterior  ex- 
tremity of  the  cranium  forms  the  rostrum,  which  supports  the  soft 
tissues  of  the  snout.  At  each  side  of  the  base  of  the  rostrum  the 
cranium  widens  abruptly.  On  the  anterior  face  of  the  widened 
portion  and  below  the  posterior  angles  of  the  rostrum  is  a  pair  of 
protruding  olfactory  capsules,  in  which  the  olfactory  sacs  are  en- 
closed. An  oval  aperture  in  the  posterior  wall  of  each  capsule 
opens  into  the  braincase  and  permits  the  passage  of  the  olfactory 
nerve  through  the  cranium. 

Back  of  the  olfactory  capsules  are  large  lateral  cavities,  the 
orbits.  The  dorsal  edge  of  the  orbit  makes  an  overhanging  ledge, 
known  as  the  supra-orbital  crest.  The  projecting  anterior  and  pos- 
terior angles  of  the  orbit  are  distinguished  as  the  prae-  and  post- 
orbital  processes. 

The  portions  of  the  cranium  back  of  the  orbit  and  at  the  sides 


27 

of  the  braincase  form  large  lateral  projections  (auditory  capsules) 
containing  the  organs  of  hearing. 

At  the  center  of  the  nearly  vertical  posterior  surface  of  the 
cranium  is  a  large  opening,  the  foramen  magnum,  through  which 
the  spinal  cord  passes. 

At  either  side  of  and  below  the  foramen  magnum  is  a  smooth 
articulatory  surface  (occipital  condyle)  articulating  with  the  cen- 
trum of  the  first  vertebra. 

The  flattened  ventral  surface  of  the  posterior  part  of  the  cranium 
forms  the  roof  of  the  mouth,  or  palate. 

In  the  mid-dorsal  line  of  the  cranium,  between  the  prae-orbital 
processes,  is  a  small  aperture  opening  into  the  brain  cavity,  the 
epiphysial  foramen.  It  is  closed  during  life  by  a  tough,  fibrous 
membrane.  The  stalk  of  the  epiphysis  extends  to  the  under  surface 
of  this  membrane. 

Between  the  auditory  capsules  is  a  deep  depression  in  the  roof 
of  the  cranium  in  the  floor  of  which  can  be  seen  the  two  small 
pores  through  which  the  ducti  endolymphatici  pass  into  the  cap- 
sules. Close  behind  them  are  two  larger  openings  for  the  perilymph 
ducts. 

A  pair  of  foramina  passes  through  the  inner  edge  of  the  prae- 
orbital  process ;  these  permit  the  passage  of  the  ophthalmic  branches 
of  the  trigeminal  and  facial  nerves  to  the  dorsal  surface  of  the 
snout.  Near  the  bottom  of  the  inner  wall  of  the  orbit  is  the  foramen 
of  the  optic  nerve.  In  the  postero-ventral  angle  of  the  orbit  is  the 
large  trigemino-facial  foramen  for  the  exit  of  branches  of  the 
trigeminal  and  facial  nerves;  in  front  of  it  is  the  small  oculo- 
motor foramen.  The  extremely  small  foramen  of  the  trochlear  nerve 
is  almost  directly  above  the  optic  foramen,  near  the  top  of  the  inner 
wall  of  the  orbit.  Close  below  the  trigemino-facial  foramen  is  the 
small  passage  for  the  abducens  nerve.  Below  the  abducens  foramen 
is  the  transbasal  canal.  Behind  and  below  the  trigemino-facial 
foramen  are  two  foramina,  through  which  pass  the  hyomandibular 
branches  of  the  facial  nerve.  The  foramen  of  the  vagus  nerve  is 
close  to  the  foramen  magnum,  upon  the  posterior  surface  of  the 
cranium.  The  foramen  of  the  glossopharyngeal  nerve  is  lateral  to 
that  of  the  vagus,  near  the  postero-lateral  angle  of  the  cranium. 

The  cranium  of  Eugaleus  is  much  like  that  of  Squalus,  except  that  the 
rostrum  is  formed  by  three  rods,  two  dorsal  and  one  ventral,  which  arise  from 
the  front  of  the  brain  case  and  converge  anteriorally  until  they  meet  and  fuse. 
The  olfactory  capsules  are  much  larger  and  of  heavier  cartilage  than  in  Squalus. 
The  auditory  region  similarly  is  more  prominent. 

(2)  The  jaws.    The  jaws  in  reality  are  the  first  pair  of  visceral 
or  gill-arches,  and  in  spite  of  the  modification  which  has  taken 
place  this  relation  can  be  seen   easily  in  the  adult  shark.     The 
upper  jaw  consists  of  a  pair  of  palato-quadrate  cartilages,  united 
medially  by  ligament,  and  bearing  the  upper  series  of  teeth.     A 
large  hooked  palatine  process  extends   from  each  palato-quadrate 
cartilage  upward  along  the  inner  wall  of  the  orbit.     The  lower  jaw 
likewise  consists  of  a  pair  of  MeckeVs  cartilages,  united  medially 
(the  union  is  called  the  symphysis),  and  bearing  the  lower  series 
of  teeth.    A  pair  of  small  labial  cartilages,  which  support  the  edges 
of  the  labial  pockets,  lie  at  each  corner  of  the  mouth. 

(3)  Visceral  arches.     The  first  of  the  visceral  arches  is  much 
larger  and  heavier  than  the  rest.     It  is  known  as  the  hyoid  arch. 
Each  side  of  the  arch  consists  of  two  rods  of  cartilage:     (1)   the 
hyomandibular  cartilage,  which  articulates  with  a  distinct  facet  on 
the  lateral  surface  of  the  auditory  capsule,  and  extends  from  here 


28 

downward,  outward,  and  backward;  (2)  the  cerato-hyal  cartilage, 
which  is  movably  articulated  to  the  hyomandibular  and  extends 
downward,  forward  and  inward.  The  ventral  ends  of  the  ceratohyals 
are  united  by  a  median,  plate-like  basihyal. 

The  palato-quadrate  and  Meckelian  cartilages  are  suspended 
from  the  hyomandibular  by  several  strong  ligaments,  the  direct  at- 
tachments of  the  jaws  to  the  cranium  being  of  soft  connective  tissue 
only.  Both  the  hyomandibular  and  ceratohyal  cartilages  bear  slen- 
der rods  (branchial  rays)  on  their  posterior  edges,  which  support 
the  anterior  wall  of  the  first  gill-pouch.  Note  the  position  of  the 
spiracle  between  the  mandibular  and  hyoid  arches.  The  anterior 
wall  of  the  spiracle  is  strengthened  by  two  small,  flat,  vertical  car- 
tilages, probably  homologous  with  the  branchial  rays  of  the  gill- 
arches. 

The  remaining  five  visceral  arches  differ  little  in  their  construc- 
tion. Dorsally,  each  has  a  flat,  sickle-shaped  pharyngo-branchial 
cartilage  attached  to  the  vertebral  column  by  fibrous  bands.  The 
pharyngobranchials  of  the  Isfet  two  arches  are  fused.  Ventrad  to 
each  pharyngobranchial  is  an  epibranchial  cartilage.  The  next 
segment  of  each  arch  is  formed  by  the  ceratobranchial  cartilage.  All 
the  epibranchials  and  ceratobranchials  except  those  of  the  fifth 
arch  bear  slender  branchial  rays.  The  ventral  ends  of  the  cerato- 
branchials articulate  with  each  other,  the  first  being  attached  to  the 
ceratohyal  by  ligament.  The  second,  third,  and  fourth  arches  have 
another  more  ventral  series  of  cartilages,  the  hypobranchials.  The 
lower  ends  of  the  hypobranchials  are  attached  to  a  large  median 
plate,  the  basibranchial.  The  fourth  ceratobranchial  joins  the  third 
hypobranchial,  while  the  ceratobranchials  of  the  fifth  arch  are  at- 
tached to  the  basibranchial  directly.  The  basibranchial  is  com- 
posed of  two  segments  closely  united  by  ligament;  the  anterior  one 
narrow,  the  posterior  broad  and  flat  in  front,  tapering  to  a  sharp 
point  behind. 

Short  teeth  of  cartilage,  called  gill  rakers,  project  into  the 
pharynx  from  the  inner  edges  of  the  arches. 

A  dorsal  and  a  ventral  series  of  extra-branchial  cartilages,  thin, 
slender  plates,  lie  on  the  external  side  of  each  gill-arch. 

PECTORAL  GIRDLE  AND  FIN.  Remove  from  the  body  the  pectoral 
girdle,  with  the  fins  attached,  and  carefully  scrape  off  the  muscles 
from  the  cartilaginous  parts.  It  will  be  found  that  the  support  of 
the  fin  is  partly  of  cartilaginous  plates  and  rods,  partly  of  horny 
fibres  (dermal  fin-rays)  which  overlie  the  extremities  of  the  car- 
tilages and  extend  to  the  edges  of  the  fin.  These  fibres  are  in  two 
layers,  one  beneath  the  skin  of  each  side.  They  are  formed  in  the 
dermis.  A  similar  arrangement  of  horny  fibres  is  found  in  all  the 
other  fins. 

The  pectoral  girdle  passes  across  the  ventral  surface  of  the  body 
and  upward  on  each  side  to  the  level  of  the  vertebral  column.  The 
stout  ventral  bar  presents  numerous  facets  for  the  origin  and  inser- 
tion of  muscles.  The  articular  surfaces  for  the  pectoral  fins  are 
well  up  on  the  sides  of  the  girdle.  The  slender  dorsal  end  of  each 
side  of  the  girdle  consists  of  a  separate  bar  of  cartilage,  movably 
articulated  to  the  lower  portion.  The  ascending  limb  of  the  girdle, 
from  the  fin  articulations  to  the  base  of  the  cartilage  just  mentioned, 
is  called  the  scapular  portion;  the  small  bar  is  the  supra-scapular; 
the  ventral  bar  between  the  fin  articulations  is  the  coracoid  portion. 

The  cartilaginous  skeleton  of  the  pectoral  fin  consists  primarily 
of  a  row  of  three  basal  cartilages,  all  articulating  proximally  with 
the  girdle.  The  middle  basal  is  much  the  largest.  Distal  to  the 


29 

basals  are  three  rows  of  rod-like  radial  cartilages,  the  proximal  row 
being  articulated  to  the  basals. 

PELVIC  GIRDLE  AND  FIN.  Remove  the  pelvic  girdle  from  the  body 
with  the  pelvic  fins  attached,  and  clean  away  the  muscles. 

The  pelvic  girdle  consists  of  an  almost  straight  bar  of  cartilage, 
slightly  thicker  at  its  middle  than  at  its  ends,  which  lies  trans- 
versely in  the  ventral  wall  of  the  abdomen.  To  each  end  is  at- 
tached a  long  basal  cartilage  which  lies  in  the  fin,  close  to  and 
parallel  with  its  inner  margin.  A  proximal  series  of  slender  radial 
cartilages  is  attached  to  the  lateral  side  of  the  basal;  a  distal  series 
of  very  short  radials  lies  outside  of  the  first  series,  while  the  portion 
of  the  fin  beyond  these  is  supported  by  the  dermal  fin  rays. 

FIRST  DORSAL  FIN.  Remove  the  mass  of  muscles  on  both  sides 
of  the  base  of  the  fin  down  to  the  vertebral  column.  The  principal 
cartilages  of  the  fin  lie  in  the  median  connective  tissue  septum 
which  separates  the  dorsal  musculature  of  the  two  sides  of  the  body. 
The  basal  cartilages  of  the  fin  are  attached  to  the  vertebral  column 
by  means  of  this  septum.  It  is  best  to  remove  the  underlying  por- 
tion of  the  column  with  the  fin.  The  cartilages  can  then  be  scraped 
perfectly  clean.  The  skeleton  of  the  fin  is  composed  of  three  rows 
of  cartilages:  (1)  a  basal  row  consisting  of  one  very  large,  flat 
plate  and  two  or  three  smaller  ones  posterior  to  it;  (2)  an  inter- 
mediate row  of  several  plates  of  nearly  equal  size;  and  (3)  a  distal 
row  of  several  very  small  plates.  The  intermediate  and  distal  rows 
extend  beyond  the  body  musculature  into  the  base  of  the  fin.  The 
remainder  of  the  fin  is  supported  by  the  dermal  rays.  In  front  of 
the  cartilages  which  have  been  mentioned  is  the  strong  spine  of 
dentine  (see  p.  5),  with  its  free  portion  sheathed  by  an  enamel- 
like  covering. 

SECOND  DORSAL  FIN.  Remove  this  from  the  body  in  the  same 
manner  as  the  first  dorsal.  Its  structure  follows  the  same  general 
plan,  the  differences  being  minor  ones  of  shape,  size,  and  number 
of  plates.  Several  thin  cartilaginous  plates  are  sometimes  formed  in 
the  median  septum  in  front  of  the  spine. 

CAUDAL  FIN.  Only  one  side  of  the  caudal  fin  should  be  cleaned, 
as  when  both  sides  are  cleaned  there  is  danger  of  breaking  the  deli- 
cate cartilages.  The  cartilaginous  skeleton  of  the  caudal  fin  con- 
sists of  a  row  of  slender  rods  along  the  dorsal  side  of  the  vertebral 
column,  extending  to  its  tip.  There  are  no  cartilaginous  elements  in 
the  fin  ventral  to  the  vertebral  column.  By  far  the  greater  part  of 
the  caudal  fin  is  supported  by  the  two  layers  of  horny  fin-rays  only. 

MUSCULATURE 

Dissect  the  skin  off  the  head,  neck,  and  body  to  back  of  the 
pectoral  fins.  Observe  first  the  musculature  of  the  dorsal  side  of  the 
neck  and  of  the  body  back  of  the  bases  of  the  pectoral  fins,  noticing 
that  it  is  composed  of  narrow,  zigzag  bands,  called  myomeres. 
Where  these  are  fully  developed  they  extend  from  the  mid-dorsal 
to  the  mid-ventral  line.  Note  carefully  the  relation  of  correspond- 
ing myomeres  of  the  two  sides,  the  exact  course  of  a  single  myomere, 
and  the  direction  of  the  muscle  fibres  in  a  typical  myomere.  Ob- 
serve also  that  the  muscles  above  the  level  of  the  vertebral  column 
form  a  thick  mass,  which  is  frequently  referred  to  as  the  dorsal 
musculature;  the  muscle  below  this  level  may  be  correspondingly 
referred  to  as  the  ventral  musculature.  As  the  muscles  described 
below  are  dissected  the  mechanical  effect  of  each  should  be  de- 
termined. 


30 

MUSCULATURE  OF  THE  HEAD  AND  NECK.  On  the  lateral  and  ven- 
tral surfaces  of  the  neck  the  primary  relations  of  the  myomeres  are 
much  modified  by  the  development  of  numerous  special  muscles, 
yet  here  and  there  traces  of  the  metameric  arrangement  still  show. 
Immediately  beneath  the  skin  is  a  thin  sheet  of  muscle  covering 
most  of  the  ventral  and  lateral  surfaces  of  the  throat  as  far  back  as 
the  pectoral  girdle.  On  the  ventral  surface  a  triangular  space  is 
left  in  front  of  the  pectoral  bar;  on  the  sides  of  the  neck  the  sheet 
extends  back  to  the  last  gill-cleft;  dorsally,  it  reaches  to  the  upper 
extremities  of  the  gill  pouches.  This  is  the  constrictor  superficialis 
muscle.  It  is  attached  to  fasciae  dorsally  and  ventrally,  and  to  the 
extra-branchial  cartilages. 

The  constrictor  superficialis  consists  of  six  metameric  segments. 
The  four  posterior  ones  are  distinctly  limited  by  the  gill-slits  and 
extra-branchial  cartilages.  The  second  is  anterior  to  the  first  gill- 
slit,  the  largest  of  all,  with  distinct  dorsal  and  ventral  portions  ex- 
tending forward  above  and  below  the  jaws.  The  first  is  recognized 
as  consisting  of  two  distinct  parts,  on  the  dorsal  and  ventral  sur- 
faces of  the  head.  The  dorsal  portion  is  a  small  curved  muscle  on 
the  anterior  wall  of  the  spiracle,  extending  from  the  external  sur- 
face of  the  auditory  capsule  to  the  inner  surface  of  the  lower  jaw. 
It  lies  close  against  the  levator  marillae  superioris  (see  below).  On 
the  ventral  surface  of  the  throat  the  posterior  constrictor  muscles 
of  the  two  sides  are  separated  by  a  large  triangular  area.  In  front 
of  this  the  ventral  portions  of  the  first  and  second  constrictors  meet 
in  a  median  aponeurosis,  from  which  their  fibres  extend  trans- 
versely, those  of  the  first  to  the  mandibular  cartilage,  those  of  the 
second  to  the  hyoidean  cartilage.  The  first  constrictor  lies  ventral 
or  superficially  to,  and  largely  covers  the  second.  Reflect  the 
first  constrictor  from  the  aponeurosis  outward,  and  demonstrate 
the  two  layers  of  muscle.  It  will  be  noted  that  the  second  to  sixth 
constrictors  consist  of  united  dorsal,  lateral  and  ventral  portions, 
while  the  first  is  reduced  to  widely  separated  dorsal  and  ventral 
parts. 

On  each  side  of  the  head,  just  outside  the  angle  of  the  mouth, 
is  a  large,  thick  muscle  arising  from  the  lateral  surface  of  the 
cranium,  and  inserted  upon  the  outer  surface  of  the  mandible,  the 
adductor  mandibularis. 

In  front  of  the  small  dorsal  constrictor  superficialis  1,  and 
scarcely  separated  from  it,  is  the  strong  levator  maxillae  superioris, 
which  extends  from  the  lateral  surface  of  the  auditory  capsule  to 
the  dorsal  edge  of  the  palatoquadrate  cartilage. 

Eugaleus.  The  dorsal  portion  of  the  constrictor  superficialis  2  reaches 
above  the  spiracle  to  the  postorbital  process.  Reflecting  it,  the  adductor 
hyomandibulae  is  seen  behind  the  spiracle,  arising  from  the  upper  part  of  the 
side  of  the  auditory  capsule  and  inserted  on  the  end  of  the  hyomandibular 
cartilage.  The  levator  palpebrae  inferioris  arises  under  the  origin  of  the  levator 
hyomandibulae,  and  passes  forward  and  downward  between  the  spiracle  and 
postorbital  process,  to  insert  in  the  posterior  end  of  the  lower  eyelid.  The 
depressor  palpebrae  superioris  arises  from  the  fascia  dorsal  to  the  spiracle, 
passes  mediad  to  the  levator  palpebrae  inferioris,  upward  and  forward,  to  insert 
in  the  posterior  end  of  the  upper  eyelid.  Remove  these  muscles.  The  infraor- 
bital  canal  passes  mediad  to  the  muscles  of  the  eyelids.  The  levator  maxillae 
superioris  lies  between  the  spiracle  and  the  orbit.  Behind  it  is  a  small  slip 
of  muscle  extending  from  the  anterior  wall  of  the  spiracle  to  the  lateral  surface 
of  the  auditory  capsule  which  seems  to  represent  the  constrictor  superficialis 
dorsalis  1. 

A  thin  sheet  of  muscle  covers  the  anterior  face  of  each  inter- 
branchial  septum.  At  the  surface  these  pass  into  the  constrictor 


31 

superficialis,  and  are  evidently  portions  of  the  latter  muscle,  though 
they  are  named  the  musculi  interbranchiales. 

Above  the  constrictor  superficialis,  lying  on  the  side  of  the 
neck  between  it  and  the  dorsal  musculature,  is  a  broad  trapezius 
muscle.  It  arises  from  the  fascia  covering  the  lateral  surface  of  the 
dorsal  musculature.  Its  fibres  pass  obliquely  downward  and  back- 
ward, mediad  to  the  posterior  gill-pouches,  to  insert  upon  the  an- 
terior edge  of  the  scapular  portion  of  the  pectoral  girdle.  The 
anterior  portion  of  the  trapezius  is  also  inserted  upon  the  epi- 
branchial  of  the  fifth  gill-arch. 

Just  in  front  of  the  mouth  is  a  pair  of  strong  muscles  (levator 
labialis  superioris) ,  each  arising  from  the  ventral  surface  of  the 
cranium  close  to  the  median  line.  They  pass  into  strong  tendons 
which  are  inserted  among  the  fibres  of  the  ventral  portion  of  the 
adductor  mandibulae.  The  muscle  mass  in  front  of  the  mouth  and 
the  lower  part  of  the  adductor  mandibularis  thus  form  the  two 
bellies  of  a  digastric  muscle,  with  the  tendon  between  them. 

*  Remove  the  ventral  portions  of  the  first  and  second  superficial 
constrictors  and  clear  the  mass  of  muscles  lying  between  the  coracoid 
portion  of  the  pectoral  girdle  and  the  mandible.  Immediately  in 
front  of  the  girdle  are  two  large  muscles,  the  coraco-arcuales  com- 
munes, whose  fibres  run  inward  and  forward.  These  muscles 
cover  the  ventral  surface  of  the  pericardium,  to  the  wall  of  which 
their  median  fibres  are  attached,  while  the  lateral  fibres  are  at- 
tached around  the  ventral  ends  of  the  gill-arches. 

In  front  of  the  coraco-arcuales  communes  are  three  large  longi- 
tudinal muscles.  The  median,  unpaired  one,  arising  from  the  fascia 
between  the  coraco-arcuales  communes  and  inserted  upon  the  pos- 
terior surface  of  the  lower  jaw,  is  the  cor aco -mandibularis.  The 
other  two,  which  lie  dorsal  to  and  outside  of  the  coraco-mandibu- 
laris,  are  the  coraco-hyoidei.  They  arise  from  the  fasciae  covering 
the  anterior  ends  of  the  coraco-arcuales  communes  and  posterior 
parts  of  the  coraco-branchiales,  and  insert  upon  the  basihyal. 

Dissect  out  the  coraco-mandibularis  and  coroaco-hyoidei,  noting 
particularly  the  form  and  place  of  origin  of  the  latter.  Dorsal  to 
the  coraco-hyoidei  are  the  first  divisions  of  the  right  and  left 
coraco-branchialis  muscles,  which  arise  from  fascia  covering  the 
anterior  ends  of  the  coraco-arcuales  communes,  and  are  inserted 
upon  the  ventral  extremity  of  the  ceratohyal  cartilage.  Notice  that 
they  pass  dorsad  to  the  anterior  branches  of  the  aorta,  and  that 
the  aorta  itself  can  be  exposed  between  them. 

Remove  the  coraco-arcualis  communis  by  dissecting  it  from 
the  pectoral  girdle  and  reflecting  it  forward.  The  other  four 
divisions  of  the  coraco-branchialis  are  now  revealed,  attached  to  the 
lateral  surface  of  the  pericardium  and  the  lateral  portion  of  the 
coracoid.  The  divisions  of  the  muscle  are  clearly  separated  only 
near  their  insertions.  The  second,  third,  and  fourth  coraco-branch- 
ials  are  attached  to  the  hypobranchial  cartilages  of  the  second,  third 
and  fourth  visceral  arches.  The  fifth  division  is  inserted  upon  the 
lateral  portion  of  the  basibranchial  and  the  expanded  medial  end 
of  the  fifth  ceratobranchial. 

The  first  aortic  branch  passes  ventral  to  the  first  coraco-branch- 
ial.  The  second  aortic  branch  passes  between  the  first  and  second 

*The  coraco-mandibularis,  coraco-hyodeus,  coraco-arcualis  com- 
munis, and  coraco-branchialis  muscles  should  be  dissected  as  a  pre- 
liminary operation  to  following  the  ventral  aorta  and  its  branches. 


32 

coraco-branchials.  The  third  aortic  branch  passes  between  the  sec- 
ond and  third  coraco-branchials.  The  fourth  aortic  branch  passes 
between  the  third  and  fourth  coraco-branchials. 

Expose  the  dorsal  ends  of  the  gill  arches  by  clearing  away 
muscles  and  other  tissues  between  the  gill  pouches  and  the  spinal 
column.  Two  sets  of  four  small  muscles  (interarcuales)  will  be 
found  connected  with  the  branchial  cartilages.  The  second,  third 
and  fourth  medial  interarcuales  extend  from  the  posterior  surfaces 
of  pharyngo-branchial  cartilages  1,  2,  and  3,  to  the  dorsal  surfaces 
of  pharyngo-branchials  2,  3,  4,  and  5.  The  first  medial  inter- 
arcuale  arises  from  the  under  surface  of  the  cranium  and  inserts  on 
the  upper  end  of  the  first  pharyngo-branchial. 

The  lateral  interarcuales  lie  immediately  below  the  medials.  The 
first  has  a  double  origin,  most  of  the  fibres  arising  from  the  lower 
part  of  the  posterior  edge  of  the  first  pharyngo-branchial;  a  smaller 
bundle  from  the  anterior  edge  of  the  second  pharyngo-branchial. 
Its  insertion  is  along  the  dorsal  surface  of  the  first  epibranchial 
cartilage.  The  second  and  third  lateral  inter-arcuales  are  like  the 
first,  but  since  the  fourth  and  fifth  pharyngo-branchials  are  fused, 
the  origin  of  the  fourth  lateral  interarcuale  is  not  divided. 

The  circular  muscles  of  the  oesophagus  are  strongly  attached  to 
the  last  pharyngo-branchial. 

MUSCULATURE  OF  THE  PECTORAL  FIN.  The  dorsal  muscle  of  the 
fin  (levator-retractor)  arises  mostly  from  the  scapular  portion  of 
the  girdle,  with  a  small  part  arising  from  the  fascia  covering  the 
lateral  musculature  of  the  body.  It  is  attached  in  fasciculi  to  the 
dorsal  surfaces  of  the  cartilaginous  rays.  The  ventral  muscle  (de- 
pressor-protractor] arises  from  the  median  portion  of  the  girdle  and 
is  inserted  upon  the  cartilaginous  rays  in  similar  fasciculi.  A  por- 
tion of  the  lateral  body  muscles  is  inserted  upon  the  scapular  por- 
tion of  the  girdle. 

MUSCULATURE  OF  THE  PELVIC  FIN.  Ventral  surface:  An  ad- 
ductor muscle  has  origin  upon  the  postero-lateral  edge  of  the  girdle; 
it  is  inserted  upon  the  antero-medial  surface  of  the  basal  cartilage 
of  the  fin.  The  depressor  muscle  consists  of  small  fasciculi,  each 
corresponding  to  a  cartilaginous  ray.  They  arise  from  the  postero- 
lateral  surface  of  the  basal  cartilage  and  are  inserted  upon  the  distal 
extremities  of  the  rays. 

Dorsal  surface:  The  abductor  arises  from  the  fascia  covering 
the  trunk  muscles,  and  inserts  upon  the  fascia  covering  the  intrinsic 
muscles  of  the  dorsal  side  of  the  fin.  These  latter  (levatores)  are 
arranged  in  exactly  the  same  manner  as  the  fasciculi  of  the  de- 
pressor. 

MUSCULATURE  OF  THE  DORSAL  FINS.  A  sheet  of  muscle  is  at- 
tached to  each  side  of  the  anterior  dorsal  fin,  extending  nearly  up 
to  the  bases  of  the  dermal  fin-rays.  This  muscle  passes  downward 
between  the  dorsal  body  musculature  of  the  two  sides.  Part  of  the 
fibres  arise  from  the  fasciae  covering  the  medial  surfaces  of  the 
body  muscles,  part  from  the  basal  cartilage  of  the  fin  itself.  They 
are  inserted  upon  the  lateral  surfaces  of  the  broad  cartilaginous  fin- 
rays.  The  muscles  of  the  posterior  dorsal  fin  are  exactly  similar  in 
arrangement. 

MUSCULATURE  OF  THE  CAUDAL  FIN.  There  is  no  special  muscu- 
lature for  the  dorsal  portion.  A  narrow,  band-like  muscle  is  found 
on  each  side  of  the  ventral  portion,  widest  above  the  triangular 
ventral  lobe.  The  fibres  of  this  muscle  arise  upon  the  flattened, 
expanded  ends  of  the  haemal  spines.  They  pass  obliquely  backward 
and  upward  to  be  inserted  in  the  fascia  underlying  the  skin. 


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